The effects of various tumor-inhibitory antibiotics on immunological responses were investigated, using serum antibody production against human gamma globulin and sheep red cells in mice, development of tuberculin hypersensitivity in guinea pigs, and skin allograft reaction in mice. The effects on antibody-forming cells in the mouse spleen were observed by JERNE'S method. Of the antibiotics examined, pluramycin significantly suppressed the immunological responses. The responses of cellular antibodies were more markedly affected than those of circulating antibodies. By the method employed, bleomycin, phenomycin, angustmycins A and C did not exhibit any significant activity of immunosuppression. The skin allograft reaction was not significantly modified by formycins A and B. Pluramycin was observed to inhibit the growth of plasmocytoma in mice.