The antitumor effects of egg-white lysozyme, at dosages between 25 and 100 mg/kg/day given for 5 to 14 days, was examined in CBA mice bearing MCa mammary carcinoma or TLX5 lymphoma. At early stages of tumor growth the antitumor action of lysozyme is statistically significant, independently from the route of administration (e.g., i.v. and oral admixed with food). With larger tumor masses, oral administration of lysozyme is effective on s.c. tumor growth but not on i.m. tumors. The effects of lysozyme in mice bearing TLX5 lymphoma consist of reduction of the capacity of tumor cells to form brain metastases: the effect is mediated by spleen cells. Dietary intake of lysozyme is also active in prolonging the survival time of animals treated with surgery and postsurgical cisplatin treatment. These effects indicate lysozyme to be an active substance, effective on the growth of malignant tumors and capable of synergizing with conventional therapies such as surgery plus cisplatin for the control of disseminated tumors in mice.