The form of von Willebrand's disease characterized by a qualitative abnormality of Factor VIII/von Willebrand factor (FVIII/vWF) in plasma has been designated as Type II. We have now identified 20 persons from five families whose qualitatively abnormal FVIII/vWF shows heightened responsiveness to ristocetin. We have classified this form of the disease as Type IIB and reclassified as Type IIA the form previously described as Type II, in which the interaction of the abnormal FVIII/vWF with platelets is decreased or absent in the presence of ristocetin. The enhanced reactivity of FVIII/vWF in Type IIB was evident in studies of ristocetin-induced platelet agglutination and of binding of FVIII/vWF to platelets in the presence of ristocetin. In both Type IIA and IIB, crossed immunoelectrophoresis of plasma FVIII/vWF demonstrated similar absence of the larger, less anodic forms. These findings suggest that ristocetin-mediated interactions between platelets and FVIII/vWF do not accurately reflect the "bleeding-time" (von Willebrand factor) defect in this newly described subtype of von Willebrand's disease.