A horse was immunized with crystallized normal human serum albumin. The antibody response paralleled that previously observed during immunization of horses with egg albumin or diphtheria toxoid. An early bleeding yielded serum which did not contain precipitins for human serum albumin, but which did inhibit the precipitation of this antigen by specific rabbit antiserum. Serum obtained from subsequent bleedings gave the typical toxin-antitoxin precipitation curve.

Various methods for quantitative estimation of human serum albumin by precipitation with horse antiserum were tried, but most proved tedious and exacting. By taking advantage of the rapid rate of reaction between human serum albumin and horse antibody and its reversibility in the zone of antigen excess, a simple, quick direct titration procedure was developed and shown to yield results which agreed remarkably well with the electrophoretic estimation of human serum albumin in fractions of human plasma and in serum or other body fluids obtained from patients.

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