The effects of the oral administration of 100 mg/kg/day of lysozyme chloride on lung metastasis development were studied in mice bearing Lewis lung carcinoma. Lysozyme was administered to mice by supplying the daily amount of lysozyme with the powdered food. Lysozyme treatment reduces lung metastasis development, by significantly reducing the number of metastases of large dimension (diameters greater than 2mm) and by causing a significant increase of the percentage of animals free of large metastases, as compared with untreated controls. Correspondingly, the same animals show a pronounced increase of the number of multinuclear giant cells in the spleen; this parameter appears to be inversely correlated with the antimetastatic effect. These effects support the hypothesis that the antimetastatic effect of orally administered lysozyme depends upon spleen activation and perhaps upon induction of multinuclear giant cells of macrophage origin. This effect is consistent with previous findings indicating the occurrence of host-mediated effects in the antitumor action of lysozyme administered through the oral route.