Male (NZW x BXSB)F1 mice spontaneously develop a disease which closely resembles human systemic autoimmune disease, involving idiopathic thrombocytopenic purpura and glomerulonephritis. We investigated whether autoimmune thrombocytopenia in the mice responded to deoxyspergualin, as immunosuppressant. Deoxyspergualin completely prevented the development of thrombocytopenia and suppressed the increase in circulating autoantibodies against platelets. This agent also ameliorated lupus nephritis. These findings suggest that deoxyspergualin may be effective in the prevention of idiopathic thrombocytopenic purpura.