Two immunoelectrophoretically defined, heretofore unidentified ß1-globulins of human serum, provisionally designated ß1C- and ß1A-globulin, were isolated by means of preparative electrophoresis and chromatography on anion exchange cellulose.

The sedimentation coefficient S020, w of ß1C-globulin was shown to be 9.5 S, and that of ß1A-globulin, 6.9 S. Both proteins were found to contain similar amounts of carbohydrate, to be devoid of lipids, and to possess the solubility characteristics of euglobulins. In the Ouchterlony double diffusion test they gave the reaction of partial identity, which revealed ß1A-globulin to be anti-genically deficient as compared to ß1C-globulin.

ß1A-globulin could not be detected in fresh sera and ß1C-globulin was absent from aged sera. Highly purified ß1C-globulin stored at 1°C. was converted to ß1A-globulin within 4 to 6 weeks, and at 37°C. was converted within 6 days. The likelihood of a dimer-monomer relationship between these two proteins was discussed.

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