Abstract 1. (1) A simple, rapid photometric method is described for the quantitative determination of the serum γ-globulin fraction. It is based upon the experimental fact that a 5:i ratio (v/v) of a 0.2% rivanol solution to serum, precipitates all the serum proteins except the γ-globulin fraction (> 85%) and a small amount of β globulins (< 15%), i.e., the transferrins. 2. (2) Since the tryptophan content of γ-globulin is high (~3.o%), application of a modified hopkins-cole reaction, specific for tryptophan, to the rivanol-containing supernatant fluid gives a purple color proportional to its globulin content. The absorbance of the purple color can be read at 560 mμ, without interference by the yellow rivanol color. 3. (3) Comparison of γ-globulin values obtained for 10 normal and 31 pathological sera with both the colorimetric and electrophoretic methods show good correlation when due allowance is made for the small amount of β-globulins present in the supernatant fluid. 4. (4) The rivanol-tryptophan method gave a mean normal value of 1.17 g% with an S.D. of ± 0.19 and a normal range of from 0.79–1.55 g%. These statistical data are almost identical with those obtained with the electrophoretic method. 5. (5) By a combination of the presently described γ-globulin method, a previously described tryptophan method for total serum globulins and the biuret method for total protein it becomes possible not only to detect diseases which result in low A/G ratios but also to distinguish the chronic from the infectious (high γ-globulin) types.