The effect of the route of administration of dimethyl sulfoxide on humoral immunity and arthritis was evaluated in the rat model of collagen II autoimmune arthritis. Intraperitoneal administration of 5 g/kg/day (days 0-12) reduced serum anti-collagen II IgG levels, delayed the onset of arthritis, but induced sterile peritonitis in all of the treated animals. The same dose given subcutaneously did not alter humoral or clinical parameters. Lower intraperitoneal doses (0.04 and 0.25 g/kg/day), although non-toxic, were similarly ineffective. Subcutaneous (5 g/kg/day) or topical treatment (both hindpaws dipped twice daily into 70% dimethyl sulfoxide) of established disease (days 16-27) produced a mild anti-inflammatory effect without any immunosuppression. We suggest that the apparent suppression of autoimmunity by dimethyl sulfoxide is dependent upon intraperitoneal administration and a toxic dose of the agent.