In human platelet lysates prepared by addition of nonionic detergent (Triton X-100) or by sonication, the multimer composition and electrophoretic mobility of platelet von Willebrand factor (vWF) were consistently modified under conditions that would favor activation of the endogenous calcium-activated, sulfhydryl-dependent neutral protease (CAP). By sodium dodecylsulfate-agarose gel electrophoresis, native platelet vWF contained some multimers that were larger than those characteristic of plasma vWF. Modified platelet vWF contained a multimer population equivalent to or smaller than that of plasma vWF plus an additional fast-migrating band. In crossed immunoelectrophoresis (CIE), modified platelet vWF was characterized by a more anodic distribution and the appearance of a distinct, cross- reactive, anodic component previously designated VIIIR:Ag fragment. In the presence of calcium, radiolabeled purified plasma vWF was also degraded by the protease in question, with a decrease in the apparent molecular weight of the reduced monomer from 230,000 to 205,000. The VIIR:Ag fragment isolated from the same degraded plasma vWF by preparative CIE was shown to be composed of an identical mol wt 205,000 subunit. Because cleavage of plasma or platelet vWF was inhibited by prior addition of leupeptin, EDTA, ethylene glycol bis (beta-aminoethyl ether)-N, N, N′, N′-tetraacetic acid (EGTA), or N-ethylmaleimide (agents known to inhibit platelet CAP) but was unaffected by numerous other protease inhibitors, including soybean trypsin inhibitor, benzamidine, hirudin, phenylmethylsulfonyl fluoride, aprotonin, or epsilon-aminocaproic acid (none of which inhibits platelet CAP), we conclude that proteolysis of vWF observed in this study is a direct effect of CAP and is not mediated by way of secondary proteases.