We have previously shown that part of the injury sustained by cold-preserved livers on reperfusion is the consequence of platelet adhesion to sinusoidal endothelium. The purpose of the present study was to determine whether prostaglandin E1 (PGE1) can reduce the injury and if so, how to maximize this beneficial effect. Rat livers were cold-preserved in University of Wisconsin solution for 30 hours then subjected to 1-hour warm ischemia after which they were reperfused at 37°C with oxygenated Krebs-Henseleit solution with or without isolated platelets. PGE1 was used to treat the donor liver during harvesting, cold preservation, and reperfusion. In some studies, PGE1 was used to pretreat platelets before exposing them to the liver, and in other studies, both liver and platelets were treated. Pretreatment of platelets with paraformaldehyde, which inactivates them, or ADP, which activates them, was also studied. Treatment of livers with PGE1 significantly decreased preservation injury when livers were reperfused in the absence of platelets. However, when platelets were added to the perfusate, prior treatment of the liver with PGE1 had relatively minor beneficial effects. Pretreatment of platelets alone with PGE1 was also beneficial, but again the effect was small. However, when both liver and platelets were treated with PGE1 there was a highly significant decrease in the extent of liver injury and platelet adhesion. Perfusate transaminase levels were lower, bile flow was improved, and histologically, livers appeared less injured. Pretreatment of platelets with paraformaldehyde produced similar results to pretreatment with PGE1. When platelets were preactivated with adenosine diphosphate, extensive hepatic injury occurred upon reperfusion despite PGE1 treatment of the liver. PGE1 can lessen preservation-reperfusion injury impressively when administered to both liver and platelets but has little effect when platelets have been preactivated.