The antitumor activity of free doxorubicin and doxorubicin entrapped in cardiolipin liposomes was evaluated in P388 ascitic leukemia, disseminated Gross leukemia, and advanced mammary carcinoma. In P388 leukemia, free drug and drug entrapped in liposomes demonstrated equivalent antitumor activity at doses of 2.2 and 4.4 mg/kg, demonstrating 52% and 69% ILS (increase in life-span), respectively. Free doxorubicin at a dose of 10 mg/kg was superior, producing a 185% ILS against 82% with liposomal doxorubicin. With an increase in administered dose the antitumor response with liposomal doxorubicin was much more pronounced; at doses of 20 and 25 mg/kg the ILS was in excess of 376%, with five of ten mice surviving tumor-free. In Gross leukemia, the optimum dose of free doxorubicin, 10 mg/kg, brought about 186% T/C (median survival in treated mice over that in controls, X 100), whereas with liposomal doxorubicin the optimum dose was 16.9 mg/kg, which yielded 214% T/C. In advanced mammary carcinoma, the maximum tumor regression with free doxorubicin was at a dose of 7.5 mg/kg, with two of six mice dying of toxicity. Liposomal doxorubicin caused maximum tumor regression at 10.8 mg/kg dose with no toxic deaths. Doxorubicin entrapped in cardiolipin liposomes was much less toxic than free drug at high doses in normal mice.