In 7 healthy volunteers, the effect of a single i.v. injection of 52 mg (7,500 IU) of an unfractionated heparin (UFH) and of 52.5 mg (5,000 anti XaU) of a low molecular weight heparin (LMWH) on coagulation parameters and platelet function has been studied. Thrombin-induced platelet aggregation was inhibited after the injection of both heparins. There was no significant change of ADP or collagen-induced aggregation after LMWH of UFH. Platelet adhesion to bovine extracellular matrix was not inhibited by UFH but was significantly reduced after addition in vitro and ex vivo after administration of LMWH. Further investigation should establish the time course of LMWH effects on platelet adhesion. A long duration of this effect could be partially correlated with the antithrombotic effects of LMWH.