The effects of the monoclonal antibody-mitomycin C conjugate against human colon cancers were studied, in vitro and in vivo. Mitomycin C (MMC) was conjugated with the human colon cancer-specific monoclonal antibody, using a cyanogen bromide method. The effect of the conjugate and free MMC, in vitro, was measured by incubation with human colon cancer SW1116 cells. The MMC concentration of the conjugate and of free MMC needed for 50 per cent killing of target cells was 0.11 microgram/ml and 7.00 micrograms/ml, respectively. The effect of the conjugate and of free MMC, in vivo, was assayed by the growth inhibition of human colon cancer xenografted in nude mice, after intraperitoneal injections of 30 micrograms MMC/kg. Antitumor effects of the conjugate against the cancer in nude mice were significantly more potent than that of free MMC.