We have examined the multimeric composition of factor VIII/von Willebrand factor in plasma and platelet lysates by means of sodium dodecyl sulfate agarose electrophoresis followed by staining with (125)I-labeled affinity-purified antibody. In normal plasma and platelet lysates, factor VIII/von Willebrand factor displayed 10 distinct multimers that ranged in apparent molecular weight from 0.86 to 9.9 x 10(6). The molecular weight difference between adjacent bands was 0.8-1.1 x 10(6). Larger material, not resolved into discrete bands, was also present with an average M(r) of 14.5 x 10(6). Though the dimer (apparent M(r) = 0.48 x 10(6)) and the monomer (apparent M(r) = 0.28 x 10(6)) generated by reduction of disulfide bonds were readily identified in this system, they were not detected in normal plasma or platelets. No differences were observed between fresh plasma prepared without anticoagulant and fresh or frozen plasma anticoagulated with either citrate or heparin. "Variant" (type II) von Willebrand's disease could be divided into two subtypes. In subtype IIA, factor VIII/von Willebrand factor in plasma consisted predominantly of the five smaller multimers with traces of the sixth and seventh (M(r) up to 4.5 x 10(6)). In subtype IIB, all these multimers were easily detected and, in addition, bands of intermediate size (M(r) = 8.5 x 10(6) and smaller) were present. In contrast, the multimeric composition of IIB platelet factor VIII/von Willebrand factor was identical to normal, whereas in subtype IIA the larger multimers were absent from platelets as well as from plasma. In subtype IIB, binding of factor VIII/von Willebrand factor to platelets occurred at lower concentrations of ristocetin than required for normal and multimers of smaller size than in normal bound. On the contrary, in subtype IIA, binding was minimal, as was true of normal factor VIII/von Willebrand factor of equivalent size. Thus, physical as well as functional differences in the two subtypes of variant von Willebrand's disease described suggest that different pathogenetic mechanisms underlie the factor VIII/von Willebrand factor abnormalities in these patients.