The oral administration of hen egg-white lysozyme to mice bearing B16 melanoma significantly reduces the formation of spontaneous lung metastases and, when combined with surgical removal of the primary tumor, prolongs the survival of the treated hosts. The antimetastatic effect, comparable with that found in the Lewis lung carcinoma and MCa mammary carcinoma systems, is independent of the direct interaction of lysozyme with tumor cells and tends to indicate the suggested intervention of an indirect action mediated by the induction of host responses.