The von Willebrand factor antigen (factor VIII-related antigen, VIIIR:Ag) multimer pattern has been analysed by SDS-agarose electrophoresis of plasmas from 116 patients (47 families) with von Willebrand's disease. In addition to previously recognized patterns, a subclassification was established between plasmas that had a type Ia pattern (VIIIR:Ag multimer pattern like that of normal plasma) and those that had a type Ib pattern in which there was a relative reduction in the concentration of the larger VIIIR:Ag multimers even though all multimeric forms were present. The different patterns were consistent within families and were inherited by autosomal dominant transmission. Von Willebrand's disease heterogeneity was apparent in the distribution of these plasmas: type Ia, 43 patients in 18 families; type Ib, 39 patients in 15 families; type II, 22 patients in 10 families, one of which was further classified as type IIB, one of which was type IIC, and three were IIA. Seven patients with severe von Willebrand's disease were also studied. In general, the interpretation of SDS-agarose multimer patterns corresponded to those previously obtained by crossed immunoelectrophoresis, but the former technique was more sensitive and could identify differences that were not apparent by crossed immunoelectrophoresis.