A blastolysin fraction, isolated from lysosyme lysates of Lactobacillus Bulgaricus strain 51 by gel chromatography, was shown to markedly inhibit the growth of sarcoma S-180. A single administration of the agent was followed by an insignificant decrease in tumor growth rate, while repeated treatment induced a complete regression of tumor in 15% of animals and resulted in a prolonged suppression of tumor growth in the others. Cells isolated from metastases which developed after the withdrawal of the drug retained susceptibility to fraction 2 of blastolysin. The latter fraction showed a similar pattern of inhibition of the growth of Ehrlich ascites tumor but did not affect hemocytoblastosis in mice LA. Histologically, fraction 2 produced tumor cell necrosis which commenced at 24 hours posttreatment. Such signs of hypercorticoidism as lysis of lymphocytes in the cortical layer of the thymus, depletion of T-dependent zones of spleen and lymph nodes and adrenal cortical hyperplasia were registered. Macrophage level in tumor was not increased. A 7-fold rise in murine serum corticosterone level at 1 hour posttreatment was established by the method of competitive protein binding.