The results from several studies examining the effects of DMSO on autoimmune phenomena have been inconclusive, possibly because of differences in experimental models, treatment regimens and doses employed. In the present investigation, autoimmune strain MRL/lpr, C3H/lpr, and male BXSB mice were placed on a continuous treatment regimen with 3% DMSO or 3% DMSO2 in the drinking water, ad libitum, commencing at 1 to 2 months of age, before spontaneous disease development could be detected. This represented doses of 8-10 g/kg/day of DMSO and 6-8 g/kg/day of DMSO2. Both compounds were observed to extend the mean life span of MRL/lpr mice from 5 1/2 months to over 10 months of age. All strains showed decreased antinuclear antibody responses and significant diminution of lymphadenopathy, splenomegaly, and anemia development. Serum IgG levels and spleen IgM antibody plaque formation, however, did not differ from control values. There was no indication of involvement of systemic immunosuppressive or antiproliferative effects, and treated animals were observed to remain healthy and vigorous with no signs of toxicity. These results demonstrate that high doses of both DMSO and its major in vivo metabolite, DMSO2, provide significant protection against the development of murine autoimmune lymphoproliferative disease. Possible mechanisms of protection are discussed.