This nephelometric assay of serum lipoprotein(a) [Lp(a)] is characterized by the use of a specific antibody to generate a high rate of light-scatter formation and the elimination of nonspecific reactions from serum samples by diluting samples in phosphate-buffered saline containing polymer enhancer polyethylene glycol (PEG), 40 g/L, and detergent before the assay. We reacted 100 microL of sixfold-diluted serum in 500 microL of buffer containing PEG with 42 microL of pure polyclonal rabbit antiserum (Dakopatts) directed against human Lp(a) and monitored the reaction by rate nephelometry with the Array Protein System nephelometer (Beckman). The standard curve for the reaction was linear in the Lp(a) range 10-1280 mg/L; antigen excess occurred between 1300 and 1400 mg/L. Calibration was performed with serial dilutions of a standard serum. Precision studies showed within-run and between-run CVs of < 2.1% and 6.9%, respectively. The nephelometric results (y) for 100 serum samples were highly correlated with those obtained by radial immunodiffusion (x) calibrated with the same materials: y = 1.07 (+/- 0.03) x - 16.2 (+/- 8.1) mg/L (r = 0.974, P < 0.001). Storing serum for 3 weeks at 4 degrees C or 3 months at -80 degrees C did not affect the results.