The interaction of beta 2-glycoprotein-I (beta 2-G-I), a plasma constituent of unknown function, with blood platelets was studied. The following results were obtained: 1) beta 2-G-I binds to washed human platelets isolated by centrifugation (WP) at one kind of specific, saturable binding sites. The dissociation constant was found to be approx. 1 X 10(-6) M. In the presence of physiological concentrations of Ca++ (2.5 mM), this specific binding is markedly reduced. Unspecific binding of beta 2-G-I to platelets, however, is not influenced by Ca++. Platelets prepared by gel filtration (GFP), differing in their in vitro aggregability from WP, exhibit no specific binding of beta 2-G-I. Binding to GFP is also not induced by activation with thrombin, collagen or ADP. beta 2-G-I causes significant alteration of the ADP-induced aggregation of GFP. Aggregation induced by thrombin, collagen, arachidonic acid or PAF-acether, however is not altered by beta 2-G-I. It is suggested, that pelleting during centrifugation causes irreversible rearrangements in the membrane of platelets.