We have evaluated the Technicon Automated Immunopreciptin System for the determination of immunoglobulins G, A and M in human serum. The new method is based on the nephelometric measurement of complexes formed during the antigen-antibody reaction. In this paper, we review the principles of nephelometry and describe its application to the measurement of antigen-antibody complexes. We have found the nephelometric method to be reliable for routine clinical use. The new method gives immunoglobulin test results which are highly reproducible, and agree well with the results determined by a radial immunodiffusion method.