The immunosuppressive activity of spergualin analogues, 15-deoxyspergualin and N-30, is presented. These compounds suppressed antibody formation and establishment of delayed-type hypersensitivity to sheep red blood cells (SRBC) in mice. Among spergualin, 15-deoxyspergualin, and N-30, 15-deoxyspergualin was most effective in suppressing immune responses. It suppressed antibody formation against SRBC by spleen cell cultures in a dose-dependent fashion without reducing cell viability. The spergualins also suppressed the mixed lymphocyte culture reaction. Peritoneal exudate cells taken from mice given immunosuppressive doses of 15-deoxyspergualin were not reduced in their functions measured: Release of lysosomal enzymes and production of superoxide anions. 15-Deoxyspergualin and N-30 were markedly effective in prolonging skin graft in rats.