An enzyme immunoassay for the determination of IgG in serum and cerebrospinal fluid was devised and evaluated to see if it performed to clinically acceptable standards. For normal levels of IgG in serum and cerebrospinal fluid the between day coefficient of variation was less than 13%. Using patient material, this new assay was compared with radial immunodiffusion; in 68 sera there was a correlation coefficient of 0.939 and in 49 cerebrospinal fluid specimens, it was 0.825. The assay range for the enzyme immunoassay was wider than that in radial immunodiffusion; the enzyme immunoassay showed greater sensitivity in that the lowest IgG value determined was 1.0 mg/l in a cerebrospinal fluid sample. No linearity aberration or matrix effects were observed. There was no appreciable interference, under the assay conditions, from immunoglobulins A or M, kappa or lambda light chains, or human serum albumin. Recoveries were about 100% over the whole assay range. The procedure has proven to be simple and relatively rapid. Assay results are available within 5 h. The reagents are stable for more than 6 months.