The cardiotoxic and myelosuppressive effects of the potential antitumor antibiotics violamycin BI (VBI) and violamycin BII (VBII) were studied in rats in comparison with daunorubicin (DAU), doxorubicin (ADR) and carminomycin (CAR). Rats were intravenously given the compounds 10 times for three weeks at total doses of 0.4 to 3 times of the acute LD50. Cardiac toxicity was measured using ECG (widening of the QRS complex), and screening parameters for bone marrow suppression were white blood cell counts, hematocrit and hemoglobin values. Following the drug injections the myocardium and the bone marrow were additionally investigated by light microscopy. With the exception of ADR, all the anthracyclines tested showed a more or less strong dissociation of cardiotoxic and myelotoxic activities. VBI was found to be only toxic on the heart but not on the bone marrow, whereas DAU, VBII and CAR were predominantly toxic on the bone marrow. Only ADR showed strong cardiac toxicity as well as distinct myelosuppression. A final evaluation of VBI and VBII has to include both the antineoplastic and the toxic properties of the compounds.