We examined the suppressive effect of a newly developed antitumor agent, 15-deoxyspergualin (DSP), on the plaque-forming cell responses to various antigens. The intraperitoneal injections of DSP at a dose of 5.0 mg/kg body weight suppressed drastically the development of plaque-forming cells in the spleen of C57BL/6 mice immunized with sheep red blood cells (0.2 ml of a 5% suspension), trinitrophenyl-lipopolysaccharide (1 microgram/mice) and trinitrophenyl-Ficoll (100 micrograms/mice). These suppressive effects were observed when the administration of DSP was started after the injection of antigens. The responsiveness of spleen cells from DSP-treated mice was also checked in in vitro culture. Responsiveness to lipopolysaccharide in vitro was not reduced. Furthermore DSP did not significantly suppress the responsiveness of spleen cells to succinylated concanavalin A or production of interleukin-1 or -2. The results are discussed with regards to the mode of action of DSP on immune responsiveness.