Type IIa von Willebrand's disease (vWd) has been characterized by the absence of the largest and a reduction in the intermediate-sized multimers of the plasma and platelet von Willebrand factor (vWf) and by the diminished response of the platelet-rich plasma of these patients to ristocetin. Other recently demonstrated abnormalities include the presence of an abnormal triplet structure of vWf. We have studied the plasma and platelets from three patients with this form of vWd and have found that both their plasma and platelets manifest the previously described abnormalities. Because of the heterogeneity of the multimeric structure of the vWf in these patients, we considered the possibility that postsynthetic events may have modified the vWf. When blood was collected in 5 mM EDTA or 5 mM EDTA/leupeptin/N-ethylmaleimide, the abnormal multimeric structure of the plasma and platelet vWf was partially normalized in that the intermediate and the largest vWf multimers were increased, the abnormal multimer structure was no longer as apparent, and the fastest migrating band (an abnormality seen only in the type IIa vWd plasma and platelets) disappeared. The enzymatic activity responsible for this degradation can be classified as a calcium-dependent protease. Studies of normal radiolabeled vWf incubated with platelet lysates from normal subjects and these patients revealed that the patients' platelets did not contain increased amounts of calcium-dependent protease activity as assessed by degradation of normal vWf. These data suggest that patients with type IIa vWd synthesize an abnormal vWf protein that is susceptible to in vitro proteolytic degradation and that proteolytic degradation can play a significant role in the phenotypic expression of vWd by modifying the plasma and platelet vWf multimeric structure.