Prodigiosin was isolated from the culture broth of Serratia marcescens B-1231. This compound inhibited the T-cell mediated immune responses such as concanavalin-A induced proliferation, mixed lymphocyte response, local graft vs host reaction and T-dependent antibody response at non-toxic concentrations. However, prodigiosin did not affect B-cell mediated immune functions such as lipopolysaccharide-induced proliferation and-activated polyclonal antibody production at the same concentrations. Prodigiosin did not cause death in vitro to lymphocytes at effective concentrations (< 100 nM) and also did not show toxicity in vivo to lymphoid organs at effective dosages (10 and 30 mg/kg). The pharmacological potencies were comparable to the activities of other T-cell specific immunosuppressants such as cyclosporin A and FK-506. In conclusion, it might be suggested that prodigiosin could be used as an immunosuppressant in clinical and immunological studies.