Immune complexes from serum were assayed for IgG by a simple rate-nephelometric method after extraction with polyethylene glycol that removes monomeric IgG. A 30-min preincubation of the extracted material in reaction buffer before the anti-IgG is introduced eliminates falsely increased values owing to precipitation that increased baseline light scatter in the reaction buffer. We found good parallelism in the reaction of anti-IgG with the IgG calibrator, aggregated human globulin, or endogenous immune complex. Thus IgG can be used for calibration in place of aggregated human globulin, greatly simplifying the assay. A good correlation was found between the present assay and the C1q-binding test (r = 0.83). The present assay is both sensitive and reproducible. The extraction and assay are straightforward and can be completed in a single morning after an overnight precipitation. The reagents for extraction are easily prepared and inexpensive, and the materials for assay are available in kit form. We believe this approach to be well suited for many clinical laboratories to measure circulating immune complexes.