UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C.  20549

Form 6-K

REPORT OF FOREIGN PRIVATE ISSUER PURSUANT TO RULE 13a-16 OR
15d-16 UNDER THE SECURITIES EXCHANGE ACT OF 1934

For the month of February

Commission File Number 1-15096

Serono S.A.

(Translation of registrant’s name into English)

15 bis, Chemin des Mines
Case Postale 54
CH-1211 Geneva 20
Switzerland

(Address of principal executive office)

Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F.
Form 20-F  ý Form 40-F  o

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(1):  o

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(7):  o

Indicate by check mark whether the registrant by furnishing the information contained in this Form is also thereby furnishing the information to the Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934.  Yes  o   No  ý

If “Yes” is marked, indicate below the file number assigned to the registrant in connection with Rule 12g3-2(b): 82-             .

SERONO’S 2005 ADJUSTED* EPS INCREASED BY 34.6%

- Significant progress in late-stage R&D programs -

Geneva, Switzerland, February 6, 2006 – Serono (virt-x: SEO and NYSE: SRA) today reported its fourth quarter and full year results for the period ended December 31, 2005.

Key Points for Full Year 2005

“2005 was a year of continued growth and business expansion.  Our improved operating leverage led to higher adjusted profit, while our aggressive business development strategy resulted in a stronger pipeline,” said Ernesto Bertarelli, Chief Executive Officer. “During 2006, we will maintain our focus on maximizing the potential of our marketed products, the efficiency of our business operations and the advancement of our most promising clinical programs.”

*  Non-IFRS financial measures are included in order to permit assessment of the performance of the company’s underlying business and include:
In Q4 2005 an additional charge of $5.5m ($5.0m after-tax) for the transfer of research activities conducted at the Serono Genetics Institute (SGI) reported in Q3 2005, a $8.0m write-down of investment in Rigel and a $1.5m write-down of investment in Cancervax.  In Q3 2005 a charge of $18.3m ($16.6m after-tax) for the transfer of the research activities conducted at the SGI from Evry, France to Geneva, Switzerland.  In Q2 2005 a $30m ($28.5m after-tax) gain on sale of investment in Celgene and a $3.7m write-down of investment in CancerVax.  In Q1 2005 a charge of $725.0m ($660.5m after-tax) related to previously reported US Attorney’s Office investigation of Serostim^® and a $4.7m write-down of investment in CancerVax.  In Q3 2004 a one-time payment of $67m ($56.6m after-tax) from a licensing agreement as well as a charge of $20.5m ($17.3m after-tax) related to the closure of a manufacturing facility. Please refer to “Adjusted net income and adjusted earnings per share” table.

- more -

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Fourth Quarter And Full Year Financial Performance

Product sales were up 7.4% to $2,338.9m in 2005 (2004: $2,177.9m), and were unchanged in the fourth quarter 2005 (2004: $604.5m) adversely influenced by the strengthening of the US dollar versus the Euro. Product sales in local currencies grew by 6.7% in 2005 and by 3.3% in the fourth quarter.  During 2005, Western European sales increased by 11.4% to $1,038.3m (2004: $931.6m), while North American sales grew by 1.2% to $848.2m (2004: $837.9m). In the rest of the world, sales grew by 10.8% in 2005 to $452.4m (2004: $408.4m).

Royalty and licensing income was $247.5m in 2005, representing 9.6% of total revenues (2004: $280.1m), and was $65.4m in the fourth quarter (2004: 75.3m).  The decrease from the prior year reflects a one-time payment of $67m from a licensing agreement in the third quarter 2004 and an additional license income totaling $11.0m for the grant of technology licenses to third parties in the fourth quarter 2004.

Total revenues increased by 5.2% (+4.5% in local currencies) to $2,586.4m in the full year 2005 (2004: $2,458.1m), and decreased by 1.4% (+1.7% in local currencies) to $669.9m in the fourth quarter 2005 (2004: $679.7m).  Excluding the one-time payment of $67m from a licensing agreement in the third quarter 2004, total revenues grew by 8.2% in 2005.

Gross margin increased to 88.6% of product sales in 2005 (2004: 86.0%), reflecting continuing manufacturing efficiency gains and a charge of $20.5m for the closure of an obsolete manufacturing site in the third quarter 2004.  In the fourth quarter 2005, gross margin of product sales reached 88.9% (2004: 87.9%).

Selling, general and administrative expenses increased 6.7% to $862.3m in 2005 (2004: $807.9m), including the launch of Raptiva^®. In the fourth quarter 2005, SG&A expenses decreased 4.5% to $223.9m (2004: $234.3m), reflecting our focus on operational efficiency.

Research and development expenses were flat at $593.6m in 2005 (2004: $594.8m). In the fourth quarter 2005, R&D expenses were $144.6m (2004: $221.3m) and included an additional $5.5m charge related to the transfer of Serono Genetics Institute, while in the fourth quarter 2004, R&D expenses included payments totaling $66.3m related to collaborations with ZymoGenetics, Micromet and CancerVax.

Reported other operating expenses in 2005 were $992.1m (2004: $239.8m) and $70.8m (2004: $70.5m) in the fourth quarter. Adjusted other operating expenses were $267.1m in the full year, excluding the $725.0m charge reported in the first quarter 2005 related to the Serostim^® settlement.

Full year 2005 operating loss was $127.5m (2004: $511.4m or 20.8% of total revenues). Adjusted* operating income was up 33.6% to $621.3m in 2005.  This significant improvement in adjusted operating margin to 24.0% of total revenues reflected steady underlying revenue growth, tight control over SG&A and R&D

* Non-IFRS earnings measures which exclude the elements set forth in the “Adjusted net income and adjusted earnings per share” table.

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expenses and improvement in gross margin.  In the fourth quarter, operating income was $163.6m or 24.4% of total revenues (2004: $80.6m or 11.9% of total revenues).

Net financial income was $40.3m in 2005 (2004: $63.3m) and $12.7m in the fourth quarter 2005 (2004: $20.7m) reflecting lower foreign exchange gains in 2005 and an unrealized gain of $8.6m in the fourth quarter 2004 related to the acquisition of ZymoGenetics’ shares.

In the fourth quarter 2005, unrealized losses of $8.0m and $1.5m were recorded in other expense, in accordance with IAS 39 revised to reflect the impairment in value of our equity stakes in Rigel and CancerVax respectively.

The reported net loss for 2005 was $106.1m compared with reported net income of $479.7m(1) in 2004.  Adjusted* net income for 2005 increased 28.4% to $565.3m.  In the fourth quarter 2005, reported net income increased 72.2%, or 70.1% in local currencies, to $144.1m (2004: $83.7m).

Basic loss per share for 2005 was $7.28 per bearer share (2004 EPS: $31.40) and $0.18 per American Depositary Share (ADS) (2004 EPS: $0.78).  Adjusted* basic earnings per share were up 34.6% to $38.80 per bearer share and $0.97 per ADS.

On April 25, 2006, the Board of Directors will propose to the Annual General Meeting a cash dividend of CHF10.0 per bearer share, CHF4.0 per registered share, and CHF 0.25 per ADS, representing an increase of 11.1% over the 2004 dividend.

For the full year ended 2005, net cash flows used for operating activities were $126.5m (2004: net cash flows from operating activities $471.7m).  The company’s liquid financial assets were $1.5 billion at the end of 2005 (2004: $1.8 billion).  As of December 31, 2005 there were 14,596,253 equivalent bearer shares of Serono S.A. issued and 14,571,273 equivalent bearer shares of Serono S.A. outstanding. At the Annual General Meeting on April 26, 2005 shareholders approved the cancellation of 962,435 bearer shares purchased by Serono under its Share Buy Back Plan.

Full Year 2006 Outlook

In 2006, Serono expects high single-digit product sales growth based on 2005 average currency exchange rates and earnings per bearer share between $42.00 and $43.40.  This outlook does not include expenses related to any new business development transactions or other non-recurring items in 2006.

(1) Previously reported amounts have been restated to reflect the adoption of new IFRS accounting standards that became effective on January 1, 2005

* Non-IFRS earnings measures which exclude the elements set forth in the “Adjusted net income and adjusted earnings per share” table.

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Key Product Review

Rebif^® full year 2005 worldwide sales were up 16.4% (15.4% in local currencies) to $1,269.8m (2004: $1,090.6m). In the fourth quarter 2005, sales of Rebif^® increased by 8.2% (11.9% in local currencies) to $335.3m (2004: $310.0m). In 2005, Rebif^® was the fastest growing multiple sclerosis (MS) therapy in the US, with sales up 31.8% to $389.5m for the full year (2004: $295.6m), and up 32.6% to $114.0m in the fourth quarter (2004: $86.0m). Rebif^® continues its market leadership outside the US with 2005 sales increasing by 10.7% to $880.3m (2004: $795.0m). Sales ex-US were $221.3m in the fourth quarter 2005 (2004: $224.0m) and increased 4.0% in local currencies.

Two key clinical trials in multiple sclerosis have made significant progress. All patients in the REGARD (REbif vs Glatiramer Acetate in Relapsing MS Disease) study of Rebif^® vs Copaxone have recently completed their first year of treatment. The Phase III CLARITY (CLAdRIbine Tablets Treating MS OrallY) study of oral cladribine is enrolling patients.

In 2005, sales of Gonal-f^® decreased by 4.5% (-5.2% in local currencies) to $547.0m (2004: $572.7m) and by 11.7% (-9.0% in local currencies) to $133.5m in the fourth quarter 2005 (2004: $151.1m), due to continued competitive pressure in the United States.  In December, Serono achieved the milestone of one million Gonal-f^® pens sold.

On January 24, 2006, Serono announced that the Japanese Ministry of Health, Labour and Welfare has approved Gonal-f^® for co-administration with Profasi^® for the induction of spermatogenesis in men with hypogonadotropic hypogonadism. Gonal-f^® will be marketed in Japan under the brand name of GONALEF^® and launch is anticipated in the second quarter 2006.

Saizen^® sales increased by 13.4% (12.5% in local currencies) to $206.5m (2004: $182.1m) in 2005, and were up 2.1% (5.9% in local currencies) to $54.3m (2004: $53.2m) during the fourth quarter. In 2005, Saizen^® was approved by the FDA in Adult Growth Hormone Deficiency and successfully completed the European mutual recognition procedure leading to marketing approval for the treatment of children born Small for Gestational Age (SGA) in 15 EU countries.

Serostim^® sales decreased to $70.4m for the full year 2005 (2004: $86.8m) and $17.0m (2004: $22.6m) in the fourth quarter.

Sales of Raptiva^® were $33.4m for the full year 2005 (2004: $4.9m), and $11.6m in the fourth quarter 2005 (2004: $3.7m). Raptiva^® is now approved in 49 countries and available in over 40 countries. Raptiva^®, the first biological to market in the European Union for the treatment of moderate to severe psoriasis provides a safe and effective alternative to conventional systemic therapies.

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Company Update

Serono announced on November 8, 2005, that Goldman Sachs has been retained to explore various strategic alternatives for the company.  This process is on-going. However, there is no assurance that any transaction will be consummated.

R&D News

Serono reports today the positive outcome of the combination phase of a study evaluating the effect of interferon-beta-1a with ribavirin for the treatment of chronic hepatitis C (HCV) in Asian patients. The proportion of patients who achieved sustained virological response, defined as an absence of detectable HCV RNA in serum after 24 weeks treatment with interferon-beta-1a in combination with ribavirin followed by 24 weeks of observation, was 57.5% (n=127).

On January 9, 2006, Serono announced the successful completion of a pivotal Phase III double-blind, placebo-controlled trial of its recombinant human growth hormone (r-hGH) in the treatment of HIV-associated Adipose Redistribution Syndrome (HARS).  This trial met all pre-specified primary and major secondary endpoints.  Serono plans to submit the file to the FDA in the first half of 2006 and expects that the data will be presented at upcoming scientific meetings.

On January 10, 2006, encouraging preliminary results from a Phase 1b clinical trial of TACI-Ig in rheumatoid arthritis were announced.  TACI-Ig appeared to be safe and well tolerated, and clear biologic and clinical responses which appear to correlate with clinical benefit were observed.  Full details from this study will be presented at a medical meeting later in the year. In addition, a Phase 1b trial of TACI-Ig in systemic lupus erythematosus is expected to finish mid-2006, and further updates related to TACI-Ig as a potential treatment for B-cell malignancies are anticipated during the second half of 2006.

In 2005 Serono added seven new projects to its R&D pipeline through new collaborations with Biomarin for Phenoptin and Phenylase, Genmab for HuMax-CD4 and HuMax-TAC, NovImmune for the fully human monoclonal antibodies, NI-0401 (anti-CD3) and NI-0501 (anti-IFN-gamma) and Rigel for the product candidates from its Aurora kinase inhibitor program, including R763.

Conference Call and Webcast

Serono will hold a conference call today, February 6, 2006, starting at 15:00 Central European Time (9:00 am Eastern Time) during which Serono Management will present the Company’s Fourth Quarter and Full Year 2005 Results.  To join the telephone conference please dial 1 866 291 4166 (from the US), 091 610 5600 (from Switzerland), 0207 107 0611 (from the UK) and +41 91 610 5600 (from elsewhere). The event will also be relayed by live audio webcast, which interested parties may access via Serono’s Corporate home page, www.serono.com. A link to the webcast will be provided immediately prior to the event.  Additionally, the webcast will be available for replay until close of business on March 10, 2006.

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Some of the statements in this press release are forward looking. Such statements are inherently subject to known and unknown risks, uncertainties and other factors that may cause actual results, performance or achievements of Serono and affiliates to be materially different from those expected or anticipated in the forward-looking statements. Forward-looking statements are based on Serono’s current expectations and assumptions, which may be affected by a number of factors, including those discussed in this press release and more fully described in Serono’s Annual Report on Form 20-F filed with the US Securities and Exchange Commission on March 16, 2005. These factors include any failure or delay in Serono’s ability to develop new products, any failure to receive anticipated regulatory approvals, any problems in commercializing current products as a result of competition or other factors, our ability to obtain reimbursement coverage for our products, the outcome of government investigations and litigation and government regulations limiting our ability to sell our products. Serono has no responsibility to update the forward-looking statements contained in this press release to reflect events or circumstances occurring after the date of this press release.

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About Serono

Serono is a global biotechnology leader. The Company has eight biotechnology products, Rebif^®, Gonal-f^®, Luveris^®, Ovidrel^®/Ovitrelle^®, Serostim^®, Saizen^®, Zorbtive™ and Raptiva^®.  In addition to being the world leader in reproductive health, Serono has strong market positions in neurology and growth & metabolism and has recently entered the psoriasis area.  The Company’s research programs are focused on growing these businesses and on establishing new therapeutic areas, including oncology and autoimmune diseases. Currently, there are more than 25 ongoing preclinical and clinical development projects. In 2005, Serono, whose products are sold in over 90 countries, achieved worldwide revenues of US$2,586.4 million. Bearer shares of Serono S.A., the holding company, are traded on the virt-x (SEO) and its American Depositary Shares are traded on the New York Stock Exchange (SRA).

For more information, please contact:

On the following pages, there are:

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Sales by therapeutic area

Sales by geographic region

Sales by therapeutic area

Sales by geographic region

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TOP TEN PRODUCTS

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Fourth Quarter Consolidated Income Statements (unaudited)

Basic Earnings per Share are calculated in accordance with IAS 33 - “Earnings per Share” by dividing the Net Income attributable to equity holders of Serono S.A., ($100.6 million) for the three months ended December 31, 2005 (2004: $58.6 million), by the weighted average number of shares outstanding during the period presented. This is 10,180,929 bearer shares (2004: 10,307,628) and 11,013,040 registere shares (2004: 11,013,040). The total weighted average number of bearer shares is 14,586,145 (2004: 14,712,844) for the three months ended December 31, 2005. As each American depositary share represents ownership interest in one fortieth of bearer share, Basic and Diluted Earnings per American depositary share is calculated as one fortieth of the Basic and Diluted Earnings per bearer share.

For Diluted Earnings per Share, the weighted average number of bearer shares outstanding is adjusted to assume conversion of all potential dilutive shares arising from outstanding stock options and the convertible bond. Diluted Earnings per Share for the three months ended December 31, 2005 includes the dilutive impact of outstanding stock options that were in the money as well as the conversion of the convertible bond that would result in the issuance of an additional 423,996 bearer shares. The number of bearer shares used to calculate the Diluted Earnings per Share for the three months ended December 31, 2005 was 10,656,766 (2004: 10,327,183).

(1)  Previously reported amounts have been restated to reflect the adoption of new IFRS accounting standards that became effective on January 1, 2005.

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Full Year Consolidated Income Statements

Basic (Loss) / Earnings per Share are calculated in accordance with IAS 33 - “Earnings per Share” by dividing the Net (Loss) / Income attributable to equity holders of Serono S.A., ($74.0 million) for the twelve months ended December 31, 2005 (2004: $341.4 million), by the weighted average number of shares outstanding during the period presented. This is 10,166,057 bearer shares (2004: 10,871,187) and 11,013,040 registered shares (2004: 11,013,040). The total weighted average number of bearer shares is 14,571,273 (2004: 15,276,403) for the twelve months ended December 31, 2005. As each American depositary share represents ownership interest in one fortieth of bearer share, Basic and Diluted (Loss) / Earnings per American depositary share is calculated as one fortieth of the Basic and Diluted (Loss) / Earnings per bearer share.

For Diluted (Loss) / Earnings per Share, the weighted average number of bearer shares outstanding is adjusted to assume conversion of all potential dilutive shares arising from outstanding stock options and the convertible bond. The effect of outstanding stock options and the convertible bond are excluded from the calculation of Diluted Loss per Share for the twelve months ended December 31, 2005 as they were anti-dilutive. The number of bearer shares used to calculate the Diluted Earnings per Share for the twelve months ended December 31, 2004 was 10,896,729.

The consolidated income statements have been extracted from the audited consolidated financial statements.

(1)  Previously reported amounts have been restated to reflect the adoption of new IFRS accounting standards that became effective on January 1, 2005 (see notes to the 2005 Financial Report for furtherinformation)

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Adjusted Net Income and Adjusted Earnings per Share