Hematin, a therapeutic agent for acute porphyrias, induces a coagulopathy characterized by thrombocytopenia and prolonged clotting times. In vitro, it activates platelets and inhibits thrombin and plasmin. Hematin has also been shown to participate in the activation of microsomal cyclooxygenase. The apparent diversity of hematin's effects on multiple hemostatic components as well as its role in prostaglandin synthesis led the authors to investigate the effects of hematin on endothelial cells and endothelial cell-platelet interactions. Marked morphologic alterations of cultured bovine aortic endothelial cells ( BAECs ) were noted after exposure to as little as 2 micrograms/ml hematin in HEPES buffer or to 40 micrograms/ml of hematin in plasma (a level achieved during hematin therapy). These changes included cell retraction and surface vesiculation. Despite the apparent severity of the changes, there was no associated cell lysis or detachment. Furthermore, the cells resumed their cobblestone appearance after hematin had been removed. Platelet adhesion to hematin-treated monolayers was significantly increased, especially when low concentrations of hematin were included in the platelet suspensions. Prostacyclin production was not increased by BAECs exposed to hematin, presumably because of the inaccessibility of hematin to cyclooxygenase in intact cells. The endothelial changes observed in tissue culture are consistent with the clinical observations of phlebitis at the injection site and provide an additional explanation for the thrombocytopenia in patients receiving hematin therapy.