The experiments on albino mice treated with rubomycin, carminomycin or dihydrocarminomycin on its 5-fold intravenous administration in doses equal to similar portions of LD50 of the respective antibiotic on its use in a single dose showed that all the 3 antibiotics induced changes in the myocardium close by their character. The heart affections were evident from swelling of separate muscle fibers, degeneration of the myofibrils, homogenization, vacuolization and resorption of the sarcoplasma, pathological changes in the nuclei, atrophy of some muscle fibers. Rubomycin had the highest cardiotoxic effect. Then followed dihydrocarminomycin and carminomycin. All the antibiotics studied in the experiments with mice had mainly an inhibitory effect on the lymphoid hemopoiesis. The effect of carminomycin was the highest. The animal death during the injections and immediately after administrations of the antibiotics must be due to their suppressing effect on hemopoiesis. The deaths at more remoted periods must be due to the cardiotoxic effect of the antibiotics.