Methodological aspects of quantitative serum protein determinations, using a newly developed laser nephelometer (PDQTM laser nephelometer, Hyland), are reported. For within-day precision, determination of the four serum proteins IgG, IgA, IgM and C'3, showed coefficients of variation between 2.5% and 4.6%. For precision from day to day, the average variation coefficient was 8%, the lowest value being 5%. Provisional normal values for the four proteins were ascertained from a small group of 60 healthy individuals. These results, as well as those obtained in states of disease, correlate with values obtained by radial immunodiffusion. It is concluded that the method is reliable, and has a number of important advantages compared to radial immunodiffusion; namely, the much higher precision, better sensitivity, particularly in lower concentration ranges, and availability of results within two hours.