As reported previously, homologous plasma lipoproteins specifically bind to the plasma membrane of human blood platelets. The two major lipoprotein-binding membrane glycoproteins were purified to apparent homogeneity and identified by their mobilities in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, both in the nonreduced and reduced state, by specific antibodies against glycoproteins IIb (GPIIb) and IIIa (GPIIIa), respectively, including the alloantibody anti-PlA1 and monoclonal antibodies. Furthermore, lipoprotein binding to intact platelets is also inhibited in a dose-dependent fashion by preincubation of the platelets with antibodies against these glycoproteins. From these experiments it can be concluded that lipoproteins bind to both components of the glycoprotein IIb-IIIa complex in isolated membranes and intact platelets. High density lipoprotein and low density lipoprotein bind to GPIIIa blotted to nitrocellulose in a way that binding of one species interferes with the binding of the other. Addition of fibrinogen significantly inhibits this binding. The specific binding of fibrinogen to GPIIIa is strongly inhibited in the presence of either of the two lipoproteins. LDL and HDL are specifically bound by isolated GPIIb, too. In our blotting experiments fibrinogen shows no binding to this membrane glycoprotein. On the other hand, fibrinogen significantly interferes with the interaction between GPIIb and the lipoproteins.