Human plasma was fractionated by ammonium sulfate precipitation, DEAE-cellulose chromatography, and Sephadex G-200 gel filtration to determine which method would give the greatest number of clearly separable kallikrein inhibitory peaks. With G-200 gel filtration three peaks could be separated which were demonstrated to contain alpha(2)-macroglobulin, C1 inactivator, and alpha(1)-antitrypsin. No other kallikrein inhibitors could be identified. The fractions containing C1 inactivator and alpha(2)-macroglobulin appeared to be more effective against kallikrein than that containing alpha(1)-antitrypsin. A patient with hereditary angioneurotic edema was shown to have an abnormal C1 inactivator protein capable of interfering with kallikrein's biologic, but not its esterolytic activity. Heat-treated human plasma, a commonly used source of kininogen for experiments with kallikrein, was shown to have kallikrein inhibitory activity.