The pharmacological activity of orally administered lysozyme, for the control of the growth of solid tumor metastases, was examined in mice bearing Lewis lung carcinoma. Groups of at least 10 tumor-bearing mice, fed daily for three consecutive weeks from subcutaneous tumor implantation with lysozyme, prepared from hen egg-white, had a pronounced reduction of the weight of their metastatic tumor to 25-50 per cent of controls within a wide range of doses (25-200 mg/kg/day). The antimetastatic effect was not related to the length of the treatment schedule employed; a short course of 7 days, given on days 1-7 after tumor implantation, proved equally active. The inhibition of the formation of lung metastases, in mice treated with lysozyme prior to tumor inoculation, lasts for at least 2 weeks after discontinuation of treatment, indicating that the antimetastatic activity observed is not associated with cytotoxic activity of the lysozyme, and is probably mediated by the elicitation of host responses. The examination of the therapeutic potential of the antimetastatic action of lysozyme supplied through the usual diet indicates that this treatment synergizes with the antitumor effects of cisplatin, given to mice after surgical removal of the primary tumor, causing a statistically significant prolongation of the survival time of the animals as compared with chemotherapy alone.