Identification of tissue factor in cultures of human skin fibroblasts permitted assessment of tissue factor activity in patients with bleeding disorders. Tissue factor activity was studied by means of a one-stage assay patterned after the prothrombin time and a two-stage test in which activation of factor X by tissue factor in the presence of factor VII was assessed. Tissue factor activity observed in cultured skin fibroblasts from five subjects with hemophilia A and B was not significantly different from that observed in pooled fibroblasts from normal controls. By contrast, tissue factor activity in cultured skin fibroblasts from four subjects with von Willebrand's disease was significantly lower than that in the control. It is postulated that the observed tissue factor defect may be related to the pathogenesis of von Willebrand's disease.