Abstract The induction of platelet aggregation by the antibiotic ristocet in requires the participation of a plasma factor, Ristocetin CoFactor (RCoF), which is deficient in patients with vonWillebrand's disease. Although it has been shown that ristocetin precipitates fibrinogen, its interaction with RCoF remains obscure. We have approached this problem by using the ultracentrifuge to examine the reaction between ristocetin and radiolabelled RCoF. RCoF was isolated from AHF concentrate and labelled with 125I using the peroxide-lactoperoxidase technique. The final product was free of other plasma proteins as determined by chromatographic and immunoelectrophoretic analysis. Immediately prior to reaction with ristocetin, aggregates in the 125I RCoF were removed by centrifugation at 35,000 RPM (110,000g). The nonaggregated protein was exposed of 1 various concentrations of ristocetin at 37°C for periods of 1 minute or 5 minutes. These