Screening tests and bioassay of von Willebrand's factor
(platelet aggregating factor) in human and animal blood plasma are
effected using a reagent of blood platelets and snake venom having
a positive platelet aggregating cofactor effect. The reagent and
tests may also suitably employ dried blood platelets. The reagent
may comprise dried platelets and either ristocetin or active snake
venom as the platelet aggregating cofactor.