Prostaglandin E1 incorporated in lipid microspheres (lipo-PGE1) was evaluated following intravenous administration as an inhibitor of ADP-induced thrombus growth rate and as a thrombus disaggregating agent following vascular damage in the microcirculation of the hamster cheek pouch. The drug was shown to be significantly more active and longer acting than PGE1 in the first experiment. Following vascular damage lipo-PGE1 was very efficacious as a thrombus disaggregating agent and was significantly more effective when infused after, rather than before, vascular damage. These data suggested that the drug might be targeted to the site of damage. When incorporated in lipid microspheres, PGE1 appeared to be more stable, longer acting and more efficacious than PGE1 alone.