Since interferon (IFN) has a mechanism of action very different from chemotherapeutic agents, it is possible that a combination of two may be of therapeutic value. The authors studied increased cytotoxic effects of anticancer drugs by IFN on human tumors xenografts in nude mice. Tumor used in this study were "SH-10", "S-7379" (gastric cancer) and "O-7294" (malignant melanoma), serially transplanted subcutaneously. IFN was injected, 5 X 10(5) mu/mouse, every day for 2 weeks and a single drug was administered 3 times every fourth day. Cytotoxic effect was determined by tumor size on day 16 after treatment. Of the 7 drugs, MMC and ADM were most effective. Other drugs showed a slight inhibition of tumor growth by combination therapy with drugs and IFN.