A method for the determination in plasma of α2-macroglobulin-bound thrombin is described. α2-Macroglobulin-bound thrombin is precipitated from plasma by 13% polyethyleneglycol, and its amidolytic activity is assayed by using the chromogenic substrate benzoyl-Phe-Val-Arg-p-nitroanilide (S 2160). After thrombin addition to plasma, only about 1.7 % of the added thrombin activity was recovered in the α2-macroglobulin precipitate. It is suggested that the contribution of α2-macroglobulin to the antithrombin activity of normal plasma is of little releva