4'-Deoxydoxorubicin has been compared with doxorubicin as regards potency, antitumor activity and toxicity in tumored and non-tumored mice treated i.v. according to different schedules. 4'-Deoxydoxorubicin was 1.5-3 times more toxic and more potent than doxorubicin. At equitoxic doses, 4'-deoxydoxorubicin was: as active as doxorubicin against Gross leukemia, mammary carcinoma and MS-2 sarcoma; slightly less active than doxorubicin against B16 melanoma; more active than doxorubicin against colon 38 adenocarcinoma. The best schedule of administration of 4'-deoxydoxorubicin in mice was the weekly treatment. The strong effectiveness against colon 38 adenocarcinoma makes 4'-deoxydoxorubicin a particularly interesting new anthracycline derivative that deserves clinical trials.