To validate the usefulness of a human tumor-nude mice xenograft system as a potential model in the secondary screening of anticancer agents, the antitumor activity of 17 anticancer drugs has been studied in the treatment of a human breast cancer tumor (MX-1) transplanted to nude mice. For evaluation of the antitumor activity of the drugs we employed the LD10 predetermined in BDF1 mice as a standard therapeutic dose. Drugs were administered IV, IP, or PO, and antitumor activity was assessed by drug-induced growth inhibition measured by caliper. Among the 17 anticancer drugs, the most active compounds (maximum inhibition rate of tumor growth: greater than or equal to 90%) are mitomycin C, chromomycin A3, vincristine, vinblastine, vindesine, and hexamethylmelamine. Another group of compounds showed moderate activity (maximum inhibition rate of tumor growth: 89%-50%), these being adriamycin, daunomycin, mitoxantrone, bleomycin, 5-FU, 6-TG, and ftorafur. The remaining four drugs (peplomycin, cytosine arabinoside, 6-MP, and methotrexate) were inactive against the MX-1 tumor. These results suggested that in the nude mouse-human tumor xenograft system of the MX-1 tumor there was a good correlation between the antitumor activity of various anticancer drugs and their clinical efficacy; this system is therefore expected to be a useful model for the secondary screening system.