The cardiotoxic effect of karminomycin and adriamycin administered intravenously for 5 times in equitoxic doses constituting equal portions of LD50 of the respective antibiotic on its single intravenous administration was studied on albino mice. Histological examination of the heart showed that almost identical damages of the myocardium occured after administration of karminomycin and adriamycin in doses of 0.45 of LD50 (1.5 mg/kg) and 0.3 of LD50 (6.3 mg/kg) respectively. The character of the damages due to the antibiotics was close, the most significant changes were observed when the animals were sacrificed 1 month after the last administration of the drug. The histological method is of value in estimation of the cardiotoxic effect of the drugs, using mice as the model suitable for the investigation. Adriamycin had more pronounced cumulative properties as compared to karminomycin: suppression of the weight gain in the mice and their death rate were higher with the use of adriamycin.