Therapeutic efficacy and preventive role of egg white lysozyme was evaluated in three types of murine ascitic tumours, namely sarcoma 180, Ehrlich's carcinoma, and Dalton's lymphoma. Lysozyme treatment produced regression of tumour growth and improved the life expectancy of the host. Growth of tumour cells treated in vitro with lysozyme prior to transplantation was also affected. In addition, lysozyme was found to have a preventive effect when administered to normal mice. The antitumour activity, therapeutic and preventive, of lysozyme seems to be due to its action on the tumour cell surface as well as on the host-mediated immune response.