von Willebrand factor-cleaving protease (vWF-cp) is a newly identified metalloproteinase. The activity of vWF-cp would vary in different physiological or pathological states. To explore activity detectron of von Willebrand factor-cleaving protease and its clinical application, the vWF-cp activity was measured by a sensitive enzyme-linked immunoadsorbent assay to detect the residual collagen binding activity (R-CBA) of von Willebrand factor before and after digestion with vWF-cp. Moreover, its activity deficiency in patients with thrombotic thrombocytopenic purpura (TTP) and solid tumors was also investigated. The results showed that the residual collagen binding assay was sensitive enough to measure the serum or plasma vWF-cp activity in 87 health individuals, 79 patients suffering from TTP and solid tumors. The coefficient of variation within and between the batches was were 3.60% and 8.35%, respectively. The serum and plasma vWF-cp activity in health individuals was (79.47 +/- 10.78)% (n = 53) and (78.79 +/- 9.17)% (n = 30), respectively, whereas the vWF-cp activity in patients with TTP, benign and malignant tumors was significantly decreased (P values were less than 0.001, 0.03 and 0.001, respectively). It is concluded that the vWF-cp activity in plasma or serum of patients with TTP and solid tumors markedly decrease, especially in patients with TTP. Assay of the vWF-cp activity using R-CBA is a simple method.