A unique class of immunosuppressive agents has been found be the use of a mouse model of immunity. A number of bensothiopyrano[4,3-c]pyrazol-3-ones are potent inhibitors of both the humoral and cellular immune response developed in mice in response to El4 tumor cells. These compounds exhibit a selectivity for inhibiting the humoral response. Structure-activity relationships and Hansch analyses of data from the mouse model are discussed. One of the most potent compounds, 2-(4'-chlorophenyl)bensothiopyrano[4,3-c]pyrazol-3-one (1, CP-17 193), exhibits ED50 values for inhibiting the humoral and cellular responses in the range of 0.5-2 mg/kg, po, in the mouse model and also inhibits antibody production to several other antigens and prolongs skin graft survival in mice. These compounds are shown in several ways to be distinct from the lymphocytotoxic drugs cyclophosphamide and azathioprine.