Summary Chromatography of human plasma on DEAE-cellulose permits the separation of 3 fractions which inhibit trypsin. The first of these inhibits trypsin only partially, and is an α2-macroglobulin. The second fraction represents most of the inhibitor in plasma, and must be largely that previously identified with the α1-globulin fraction. The third fraction is very stable to heating at low pH, and is largely made up of material with a mobility very close to that of albumin. A fourth component can be shown to differ from these with respect to its behavior in gel filtration.