A quantitative assay based on endpoint immunonephelometry was developed for human apolipoprotein A-II (apoA-II) in plasma or serum. Dilution of plasma samples with a 0.1 mol/L solution of sodium cholate enhanced the quantification. We used either purified apoA-II as the primary standard or plasma as a secondary standard. Results correlated well (r = 0.90) with those by a double-antibody radioimmunoassay for 63 serum samples from both normal and hyperlipemic individuals. The interassay coefficient of variation for the immunonephelometric assay was 7% within a working range between 0.05 and 0.7 microgram of apoA-II per sample (corresponding to a 1500-fold final dilution of serum). No extraction of samples with organic solvent is necessary if the triglyceride concentration is less than 4 g/L.