Human tumors transplanted into nude BALB/c-nu mice have been used to test the sensitivity of the tumors to various anticancer agents. Three cancer cell lines from the stomach and one from the colon were transplanted into nude mice to establish a standard assay method for selection of effective drugs on individual tumors using the criteria of the Japanese Association of Sensitivity Determination for Carcinostatic Agents. From the LD values of anticancer drugs in nude mice, the appropriate doses of drugs were 6 mg/kg of MMC X 1(i.p.), 50 mg/kg of 5-FU q4d X 3 (i.p.), 120 mg/kg of CPA (i.p.), 30 mg/kg of ACNU (i.p.) 8 mg/kg of CDDP (i.p.) and 8 mg/kg of ADM (i.v.). At 3 weeks after initial treatment, the inhibition rate (IR) of the tumor was calculated from the formula IR = (1-T/C) X 100%, where T is the mean tumor weight of the treated group and C is the mean weight of the untreated group at that time. The tumors respond well to the anticancer agents when IR is more than 58%.