The breakdown products of the complement protein C3 function in receptor-mediated immune clearance. The catabolism of the C3 molecule, mediated by factors I and H, results in the generation of the fragments C3c and C3dg. C3dg binds to human platelets in a specific and saturable manner. The direct interaction of platelets with soluble C3dg may contribute to immune-mediated platelet destruction. More importantly, platelets may interact with opsonized pathogens or complement-activating immune complexes via C3dg. In this report, we investigated the interaction of C3dg with platelets and calculated the Ka to be 3.2 x 10(6) M-1 with 1100 to 2200 specific binding sites/platelet. In the presence of 5 mM calcium, both the Ka and the number of specific binding sites were modestly decreased to Ka = 2.8 x 10(6) M-1 with 1400 to 2400 specific binding sites/platelet. The Scatchard plots demonstrated a curvilinear character. On labeling C3dg with peroxidase and visualizing platelet-bound C3dg by electron microscopy, it was shown that binding sites for C3dg were restricted to the platelet plasma membrane. Using a cell attachment assay, platelet adhesion to C3dg was readily detectable; attachment to C3dg-coated plates was not blocked by fibrinogen or fibronectin. We have characterized the C3dg-binding protein of platelets using the chemical cross-linkers, SASD and DSS, to cross-link C3dg to thrombin-stimulated platelets. Gel filtration of the 125I-labeled C3dg-platelet complex revealed the presence of a large protein complex that was absent when 125I-labeled C3dg alone was analyzed. SDS-PAGE of the radiolabeled cross-linked protein, followed by autoradiography, identified a 95-kDa membrane protein. The relationship of this C3dg-binding protein to other platelet membrane proteins has yet to be determined.