We investigated the synergistic anti-tumor effects of K-18 (conjugate of human gamma-globulin and melphalan) and concomitantly administered hyperthermia on Lewis lung carcinoma in mice. The antitumor effect of a subeffective dose of K-18 or an equivalent dose of melphalan alone was enhanced by local hyperthermia. K-18 administration demonstrated a greater potentiation for inhibition of tumor growth than that of melphalan alone. Local hyperthermia plus K-18 reduced the dose of melphalan required for tumor growth inhibition by melphalan alone. The combined application of K-18 and hyperthermia for treating cancer warrants further study.