The tumor-modifying effect of cardiogen peptide was studied on rats with transplanted M-1 sarcoma. The level of apoptosis of tumor cells after cardiogen injections in all experimental groups was higher than in the control. The dose-dependent inhibition of M-1 sarcoma growth after injection of cardiogen was caused by the development of hemorrhagic necrosis and stimulation of tumor cell apoptosis. The parameters of proliferative activity indicate that inhibition of tumor growth was not caused by the direct cytostatic effect of the drug on the tumor. Morphological signs indicate a specific mechanism of cardiogen action, realized through the vascular network of the tumor.