The toxicity and antitumor activity of liposomal vincristine preparations have been examined. Vincristine was encapsulated inside egg phosphatidylcholine (EPC)/cholesterol (55/45, mol/mol) and distearoylphosphatidylcholine (DSPC)/cholesterol (55/45, mol/mol) vesicles utilizing transmembrane pH gradient (inside acidic) drug uptake processes. Trapping efficiencies approaching 100% were achieved for this procedure using drug:lipid ratios as high as 0.2:1 (w/w). Although both EPC/cholesterol and DSPC/cholesterol liposomal systems yielded high trapping efficiencies, DSPC/cholesterol vesicles exhibited superior drug retention properties. This ability to retain entrapped vincristine was related to maintenance of the transmembrane pH gradient as well as the membrane permeability properties. Thirty-day dose-response survival studies in mice indicated that vincristine encapsulated in DSPC/cholesterol liposomes was less toxic than free drug. The 50% lethal dose of 1.9 mg/kg in CD-1 mice observed for free vincristine increased to 4.8 mg/kg upon administration of the drug in liposomal form. Liposome encapsulation of vincristine also enhanced the antitumor activity against murine P388 and L1210 lymphocytic leukemia models. This resulted from increased efficacy for liposomal vincristine at doses equal to free drug (liposomal/free drug median survival times greater than 1.0) as well as the ability to administer increased doses of liposomal vincristine. The combined effects of decreased toxicity and increased antitumor efficacy of liposomal vincristine over free drug suggest significant clinical utility of appropriate liposomal vincristine systems.