Topotecan is a promising anticancer agent presently undergoing clinical evaluation worldwide. Topotecan, camptothecin, 9-aminocamptothecin, and CPT-11 have aroused considerable interest in recent years for their ability to halt the growth of a wide range of human tumors. For each analogue an important structural requirement for biological activity is a closed alpha-hydroxy lactone ring moiety. Unfortunately, this functionality hydrolyses rapidly in aqueous solution under physiological conditions (i.e. pH 7 or above), resulting in an inactive carboxylate form of the drug. In this report, we demonstrate that topotecan's half-life in human plasma (pH 7.6) can be enhanced dramatically by packaging the drug within the aqueous, pH 5-adjusted confines of lipid vesicles composed of diasteroylphosphatidylcholine. We have also demonstrated that drug sequestration within the liposomal particles can be efficiently accomplished. Thus, our preliminary experiments suggest that liposomes may be of potential utility for markedly improving the stability of topotecan in circulation.