Samples of platelet-rich plasma (PRP) and platelet-poor plasma (PPP) obtained from the differential centrifugation of blood from normal donors anti-coagulated with 3.8% sodium citrate were treated with ristocetin. The mixtures were incubated at 37.degree. C for 5 min with stirring, followed by centrifugation and the resulting plasmas were assayed immediately for factor VIII clotting (VIII:C) activity, VIIR:WF [von Willebrand factor] and factor VIII-related antigen (VIIIR:Ag). Ristocetin at 2 and 5 mg ml-1 interacted with the factor VIII complex with the greater consumption of VIIIR:WF than VIII:C in the presence of platelets. Ristocetin 1 mg ml-1 induced aggregation of PRP but the aggregation response was variable. It may be that VIIIR:WF was also consumed by ristocetin at this concentration and that the technique used to assay this activity was not sufficiently sensitive to reveal the small changes that occurred.