The authors have developed an improved assay for von Willebrand factor (VIIIVWF) utilizing 4% dimethylsulfoxide (DMSO) to freeze formalin-fixed washed platelets. A platelet aggregometer is used to conduct the assay. The platelets are thawed and an activity curve is established using normal pooled plasma to support ristocetin-induced agglutination. The VIIIVWF level in unknown plasma can then be measured. The platelet preparation is stable for more than a year. Correlation of VIIIVWF measured by this system and Factor VIII antigen (VIIIAg) measured by electroimmunodiffusion was excellent. This is evidence that these two properties reside on the same molecule. Neither VIIIVWF nor VIIIAg correlates well with Factor VIII antihemophilic factor (VIIIAHF).