Measurement of immunoglobulins (Ig) and of complement component C3 in human serum by automated and manual nephelometric techniques is tedious, and the effective linear range is too narrow. We describe a laser nephelometer/reagent system for measuring serum proteins by the use of forward light scatter (which enhances the ratio of reaction/blank), electronic blank subtraction, laser light (632 nm), and electronic signal selection. We report data establishing the range of linearity for immunoglobulins IgG, IgA, IgM, and complement C3 with this system, and correlations with results by a radial immunodiffusion. We also compared an electroimmunodiffusion system for quantitation of albumin and IgG in cerebrospinal fluid to our technique. The nephelometric system described provides a rapid, accurate, precise, and objective way to measure immunoglobulins and C3.