Mitomycin C adsorbed onto activated carbon particles (MMC-CH) has been administered intraperitoneally for C57BL/6 mice. The weight of the spleen and thymus of the mice given MMC-CH was decreased lesser than those of the mice given mitomycin C aqueous solution (MMC-AQ). The number of peritoneal exudate cells (PEC) in the mice given MMC-AQ was decreased remarkably on 1st day after MMC-AQ administration and recovered within normal range on the 7th day. On the other hand, the number of PEC in the mice given MMC-CH was increased remarkably on the 1st day and then gradually decreased to normal range on the 7th day. Reactivity of spleen cells by Con A was inhibited in the spleen cells from the mice given MMC-AQ more than those from the mice given MMC-CH. Fifth percent lethal dose was 8.0mg/kg in the mice given MMC-AQ, and 18.2mg/kg in the mice given MMC-CH.