1. Human plasma contains a variety of proteins that are capable of inhibiting plasmin activity. Whole plasma possesses 'rapid' and 'progressive' plasmin-neutralizing activity: this study assesses the contribution of individual protease inhibitors to this plasmin-neutralizing property of plasma. 2. Rapid and progressive antiplasmin activities of human plasma correlate with alpha2-macroglobulin and alpha1-antitrypsin concentrations respectively. 3. Fluctuations in the amounts of the other measured inhibitors (antithrombin III, Cl in activator and inter-alpha-tryspin inhibitor) did not influence the measured antiplasmin activity.