Porcine or bovine factor VIII concentrates (FVIII:C) have been used during the past 3 decades to control bleeding in patients who have developed antibodies to human factor VIII. Since current preparations of animal FVIII:C are not known to transmit infectious agents such as hepatitis or human immunodeficiency virus, they are of potential therapeutic interest. A purified porcine FVIII:C (Hyate:C) is now widely used as an alternative to human FVIII:C in patients with inhibitor. Unlike earlier preparations of porcine FVIII:C, thrombocytopaenia is rare with the current preparation. Nonetheless, it causes the aggregation of human platelets in vitro. Our aim was to identify precisely the plasma factor which induces platelet aggregation. The effects of commercial porcine FVIII:C, porcine fibrinogen, porcine fibronectin and the corresponding preparations from human origin on platelet aggregation were studied. Platelet aggregation was quantified by measuring the fall in single platelet count in human whole blood. Of these preparations, only porcine FVIII:C (0.1-1 U/ml) and porcine fibrinogen (80-600 micrograms/ml) induced a fall in single platelet count of up to 85% due to aggregation. The extent of aggregation was directly proportional to the amount (0.007-0.1 U/ml test aliquot) of residual von Willebrand factor antigen (vWf:Ag) in the preparations. A monoclonal antibody to vWf:Ag inhibited the aggregation. We believe that the aggregation of human platelets induced in vitro by porcine FVIII:C is mediated by vWf:Ag which also may be responsible for thrombocytopaenia reported following administration of porcine FVIII:C in vivo.