The new anti-tumor substance 15-deoxyspergualin was shown to have immunosuppressive properties. Immunosuppression seemed to be predominantly directed against mononuclear phagocytes and to monocyte dependent lymphocyte effector functions. The substance was shown to be effective in the prevention of the rejection of allogeneic transplants. This was shown in a rat tail skin transplantation model (LEW----Fischer). Prolongation of graft survival time was increased about two fold after either i.p. or p.o. administration of 15-deoxyspergualin. Suppression of monocyte activity would thus be a novel mode of action for an immunosuppressive substance being effective in organ transplantation.