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 α₂-macroglobulin, C1̄ inactivator, and α₁-antitrypsin. No other kallikrein inhibitors could be identified. The fractions containing C1̄ inactivator and α₂-macroglobulin appeared to be more effective against kallikrein than that containing α₁-antitrypsin. A patient with hereditary angioneurotic edema was shown to have an abnormal C1̄ 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.