Ristocetin induces platelet agglutination in the presence of human factor VIII-associated ristocetin cofactor (vWF). The specificity, extent, and tenacity of binding among these reactants during agglutination and deagglutination were examined. Purified human vWF polymers were radioiodinated and reisolated. Radioiodinated vWF, a disulfide-linked polymer of 230,000 dalton subunits, attached to formalinized human platelets only in the presence of ristocetin. This binding reached equilibrium within 30 sec, and as ristocetin concentrations were raised from 0.2 mg/ml, the extent of attachment increased progressively to reach maximum at 0.5 to 0.6 mg/ml ristocetin. Ristocetin-induced binding was inhibited by vancomycin, unlabeled-purified vWF polymers, normal and hemophilia A plasma, and rabbit anti-human vWF. Binding was not impaired by plasma without detectable vWF or by naturally occurring human IgG antibodies to factor VIII coagulant activity. When formalinized platelets were pelleted from suspensions containing 125I-ristocetin, small quantities of radiolabeled ristocetin associated with platelets both in the presence or absence of vWF. About 95% of the attached 125I-ristocetin was removed by subsequent washes in buffered saline. The attachment of unmodified ristocetin or 125I-ristocetin to platelets, or the formation of complexes with vWF, could not be detected by agarose column chromatography, sucrose cushion ultracentrifugation, or equilibrium dialysis. These results indicate that (1) the initial binding of human vWF polymers to platelets is a specific interaction which requires the presence of ristocetin; (2) ristocetin and human vWF do not form persistent complexes in solution; and (3) the association of ristocetin and platelets is of low affinity.