A method for the differential determination of plasma antithrombins, antithrombin III and alpha2 macroglobulin, is described. The method is based on the selective inactivation of plasma alpha2 macroglobulin by treatment with 0-1 M methylamine for 10 minutes at 37 degrees C and on the observation that antithrombin III and alpha2 macroglobulin inhibited in defibrinated plasma low concentrations of thrombin without mutual interference and according to pseudo-first order reaction. In healthy subjects antithrombin III was shown to account for about 70% of the total antithrombin activity. But in patients with liver cirrhosis, where low levels of total antithrombin activity were observed, the relative contribution of antithrombin III was found to be noticeably lower.