A helium-neon laser nephelometer prototype manufactured by Hoechst Behring Institut was evaluated and used in our laboratory during a two-month period for the measurement of IgG, IgA and IgM. Two methods of dilution were usedto prepare standard curves. The best results were obtained when the patient serum was diluted one hundred fold (10-100 microliter) and incubated for 15 min at room temperature in a disposable plastic cuvette with the specific antiserum diluted five fold (100-300 microliter). The intensity of the light scattered by the antigen-antibody complexes was then immediately read on the digital voltmeter. The working ranges were 186-5944 mg/dl for IgG, 33-1066 mg/dl for IgA and 17-545 mg/dl for IgM. Standard curves obtained for the three immunoglobulins on six different days could be superimposed perfectly demonstrating an excellen reproducibility. The coefficient of variation for IgG (n = 8) was 4.1% at 5800 mg/dl. The results (n = 37) produced by the laser nephelometer (y) correlated very well with those obtained by radial immunodiffusion (x) for: IgG y = 8.07 + 1.00 X, r = 0.997, IgA y = - 6.51 + 1.05 X, r = 0.996, IgM y = 3.35 + 0.98 X, r = 0.998.