Purified porcine Willebrand factor was analyzed by agarose-sodium DodSO4 electrophoresis. Multiple forms of the protein were found in a series of increasing molecular weights. A molecular mass calibration curve was constructed with fibrinogen (3.4 X 10(5) daltons), IgM (1 X 10(6) daltons), and glutaraldehyde-crosslinked IgM polymers (2, 3, and 4 X 10(6) daltons). As measured by this procedure, the apparent molecular weight of Willebrand factor polymers ranged from 1.1 X 10(6) to 2.1 X 10(7). Each member of the series differed from one another by approximately 1.5 to 1.9 X 10(6) daltons, indicating that members of the series were polymers of 6-mers to 8-mers of the 2.3 X 10(5) dalton subunit. Various purification procedures, used to isolate Willebrand factor active in inducing platelet aggregation, were seen to fractionate the polymers, in part, on the basis of size. The same purification procedures, when applied to procine von Willebrand plasma, failed to yield protein of molecular weight greater than 1.1 X 10(6).