The activities of established and experimental antitumor drugs were tested against the BRO and MeWo human melanomas transplanted in the subrenal capsule of phenotypically immunocompetent (CBA × C57BL/6)Fi mice. Immunosuppression was achieved with preliminary whole-body radiation, and cytological examination showed that the tumors of untreated mice consisted largely of viable tumor cells. Several drugs tested showed moderate activity against one or both tumors. The system described provides a basis for testing anticancer agents against a panel of human melanomas implanted in immunocompetent mice