Assay and characterization of the factor in porcine and bovine plasma which aggregates human platelets.

A sensitive and objective bioassay has been developed to measure the factor present in porcine or bovine plasma which aggregates human blood platelets. The technique is based on direct measurement of platelet aggregation using an electronic particle counter. Using this assay it has been established that platelet aggregating activity is present in both bovine and porcine plasma and in therapeutic concentrates of factor VIII prepared from these materials. Platelet aggregating activity was found to be a function of the factor-VIII-related antigen concentration and independent of factor-VIII coagulant activity. The activity is absent in the plasma from pigs with von Willebrand's disease. It is concluded that factor-VIII-related antigen and the platelet aggregating factor are closely related and may represent one protein species.