Systems of blood coagulation in patients treated with antibiotics of the anthracycline group were studied. Rubomycin was used in the treatment of patients with acute leukemia Adriamycin and carminomycin were used in the treatment of patients with solid tumors. The antibiotics affected the process of blood coagulation mainly through their cytostatic effect on thrombocytopoesis. Thrombocytopenia induced deficit of thrombocytal factors participating in the process of blood coagulation which resulted in hypocoagulation and hemorrhagic complications. The plasmic factors did not significantly change during the antibiotic therapy. A tendency to decrease in the levels of prothrombine, fibrinase and fibrinogen was noted which was possible due to an inhibitory effect of the antibiotics on the function of the reticuloendothelial tissue cells or indirectly to suppression of the tumor process. More pronounced changes in the system of blood coagulation of patients treated with rubomycin were probably associated with inferiority of the thrombocytal apparatus of the patients with acute leukemia treated with the antibiotic.