Abstract Factor VIII coagulant activity was separated by ion-exchange chromatography from the von Willebrand's antigen (Factor VIII-like antigen). The von Willebrand's antigen was also separated from additional contaminating proteins and no longer aggregated washed human platelets in the presence of the antibiotic, ristocetin. Recombination of the von Willebrand's antigen with these proteins restored restocetin-induced platelet aggregation. Highly purified human fibrinogen also restored ristocetin-dependent platelet aggregating activity to the purified von Willebrand's antigen. The separated Factor VIII coagulant activity was unstable and lost >98% of the coagulant activity within three days at 0–4 °C. These findings show that Factor VIII preparations which demonstrate ristocetin-induced aggregation of washed human platelets contain at least three separable components.