The differential effects of the i.v. administration of egg-white lysozyme on primary tumor growth and on the formation of spontaneous and artificial lung metastases have been determined in mice bearing two rodent metastasizing tumors: Lewis lung carcinoma and MCa mammary carcinoma. The depression of metastasis formation was particularly pronounced at 50 and 100 mg/kg/day given on days 1,5,10,15 after tumor transplantation, causing a correspondent prolongation of the life-span of the animals carrying artificial induced lung metastases. Contact between tumor cells and egg-white lysozyme seems at least partially responsible for the observed antitumor effects, although no direct cytotoxicity for tumor cells has been detected yet.