A long term incubation in NBT-solution leads not only to NBT-reduction by polymorphnuclear leukocytes and monocytes, but also in washed human platelets. The reaction is enhanced by ADP and to a smaller and very variable extent also by Thrombin and Ristocetin. It is inhibited by incubation in plasma environment as well by DBcAMP and PGE1. After a detailed demonstration of the applied method, some possible biochemical mechanisms of the NBT-reduction in human platelets are discussed.