A simple gel diffusion method for the measurement of gamma-globulin in serum has been presented. It can also be adapted for gamma-globulin determinations in cerebrospinal fluid. The accuracy of this method is greater at low concentrations of gamma-globulin, and it is primarily useful as a screening test for hypogammaglobulinemia. Sera of 50 normal children aged 5 to 18 years were examined by this method. The range of gamma-globulin was 0.9 to 2.2 gm/100 ml, with a mean of 1.41. Comparison with serum concentrations on normal children determined by electrophoresis indicates that the values for the gel diffusion method are slightly higher.