Bactobolin prolonged survival period of mice bearing leukemia L-1210 in various dose schedules. The administration of bactobolin before or at time of immunization with sheep red blood cells (SRBC) did not affect antibody formation and delayed-type hypersensitivity (DTH) to SRBC. The administration after immunization suppressed antibody formation markedly but not DTH response. Bactobolin showed stronger suppressive action on antibody formation in vitro than mitomycin C. Bactobolin did not reduce establishment of tumor immunity which was mediated by T cells and macrophages. Comparing to other antitumor antibiotics which were effective against L-1210, bactobolin did not affect phagocytosis of mouse peritoneal macrophages. It has an extremely low toxicity to mouse spleen cells treated by concanavalin A (Con A) and lipopolysaccharide (LPS). It did not affect colony formation of mouse bone marrow cells in the presence of LPS-induced colony stimulating factor. The administration of bactobolin did not reduce the number of leucocytes in peripheral blood. From these results, the usefulness of bactobolin in the treatment of cancer was discussed.