A simple immunoturbidimetric method for quantifying apolipoproteins (apo) A-I and B in serum or plasma is described. A special reagent formulation, including large amounts of suitable detergents, obviates the need for a sample blank even with grossly lipemic specimens. The assay is rapid, easily automated, and thus convenient for routine work. For both apo A-I and apo B, the assay range is about 0.2-3.5 g/L. The performance characteristics were assessed with discrete (Optimate and Olli CD) and centrifugal analyzers (Cobas Fara and IL Monarch 2000). Average analytical recovery was 101.5% for apo A-I and 99.4% for apo B. Dilution tests showed found/expected ratios of 101.2% (apo A-I) and 101.0% (apo B). Overall precision (CV) ranged from 1.4% to 3.3% for apo A-I and from 1.1% to 8.3% for apo B. Comparisons with commercially available rate nephelometry, radial immunodiffusion, and immunoturbidimetric methods gave good correlations (r greater than or equal to 0.938). Using the immunoturbidimetric method, we also established the relationships between apolipoproteins and lipids and determined the reference intervals. We conclude that the proposed method is suitable for routine use in clinical laboratories.