A new dosage form of mitomycin C (MMC-CH) comprising 0.75 mg/ml of activated carbon particles adsorbing mitomycin C at 124 micrograms/mg and 7 micrograms/ml of mitomycin C in a free state was injected intraperitoneally to male rats of Donryu strain transplanted intraperitoneally with 10(7) cells/rat of Yoshida sarcoma 4 days before injection. The rats were subjected to autopsy within 60 min after injection. MMC-CH adhered selectively to the tumor surface of Yoshida sarcoma growing intraperitoneally rather than to the surface of organs such as small intestines which the surface cancer did not affect. Within 120 min after injection, mitomycin C concentration in the tissue samples was bioassayed. Intraperitoneally injected MMC-CH distributed high levels of mitomycin C to the tumor rather than to the unaffected organ located intraperitoneally. Animals killed by cancer after the treatment of MMC-CH were autopsied. Histological effects of MMC-CH were studied microscopically. Mitomycin C adsorbed on activated carbon particles induced degenerative changes in the tissues of tumors to which the activated carbon particles had adhered.