The antitumor activity of "blastolysin", a preparation that consisted mainly of glycopeptide fragments of the cell wall of Lactobacillus bulgaricus, was studied. Blastolysin was found to exert a specific antitumor effect on sarcoma S-180, leukemia P-388, plasmacytoma MOPC-315, adenocarcinoma AKATOL, melanosarcoma B-16, carcinoma LIC, and spontaneous tumors in mice. It is of low toxicity, does not inhibit hemopoiesis, and its action on tumor tissue differs considerably from that of the known antitumor chemotherapeutic agents.