Activation and proliferation of lymphocytes requires the active signal transducer Ras. Activation of lymphocytes, associated with autoimmunity, may therefore be modified by S-farnesylthiosalicylic acid (FTS), a synthetic substance that detaches Ras from the inner cell membrane and induces its rapid degradation. The MRL/lpr mouse is a genetic model of a generalized autoimmune disease sharing many features and organ pathology with systemic lupus erythematosus (SLE) and the primary antiphospholipid syndrome (APS). The objective of the present study was to examine the effect of FTS on laboratory and clinical pathology in the MRL/lpr mouse. Female MRL/lpr (n = 50) and MRL/++ control (n = 35) mice were treated intraperitoneally with either FTS (5 mg/kg/day) or saline between 6 and 18 weeks of age. The mice were weighed, tested for proteinuria and lymphadenopathy, lymphocyte proliferation, antibodies, grip strength and behaviour in an open field. FTS treatment resulted in a 50% decrease in splenocyte proliferation to ConA, LPS and a disease specific antigen, beta(2)-glycoprotein-I, and in a significant decrease in serum antibody levels against cardiolipin and dsDNA. Proteinuria and grip strength were normalized and lymphadenopathy and postmortem lymph node and spleen weights were significantly reduced in FTS treated MRL/lpr mice. These findings indicate that modulation of Ras activation has a significant impact on the MRL/lpr model and may represent a new therapeutic approach for the treatment of systemic autoimmune diseases such as SLE and APS.