von Willebrand factor (VWF) does not normally interact with platelets in the bloodstream. Binding to exposed vascular subendothelium, however, enables VWF to interact with the platelet glycoprotein Ib-IX-V complex (GP Ib-IX-V). This change in function may reflect a change in its conformation. Ristocetin also promotes interaction of VWF with GP Ib-IX-V; it thus provides a model for changes in VWF conformation and function that may occur in vivo. The fluid-phase conformation of VWF was evaluated from its susceptibility to proteolytic digestion. Ristocetin markedly altered the pattern of VWF digestion by trypsin, increasing the prevalence of two major proteolytic fragments (109 and 160 kDa), and decreasing that of four fragments (130, 145, 181 and 199 kDa). Vancomycin, a structurally related antibiotic, did not affect the digestion pattern. However, it partially reversed the ristocetin-induced change in digestion. Changes in prevalence of five of the tryptic fragments of VWF with ristocetin and vancomycin correlated closely with changes in VWF binding to GP Ib-IX-V. Heparin also partially inhibited the ristocetin-induced changes in tryptic digestion of VWF. These observations suggest that ristocetin may modulate VWF conformation in such a way as to expose its GP Ib-binding domain and enable it to interact with the platelet. Such modulation also exposes a cryptic site (or sites) for proteolytic cleavage by trypsin.