A sensitive and specific enzyme-linked immunosorbent assay (ELISA) for nonhuman primate serum apolipoprotein A-I (apoA-I) is described. The assay is a noncompetitive, sandwich ELISA in which polystyrene microtiter plates were used with purified, monospecific goat anti-monkey apoA-I antibodies adsorbed on the wells. The serum samples were added to the coated wells, incubated, and after washing, antibodies conjugated to horseradish peroxidase were added. After further washing, the bound label was assayed. A heat treatment step, 52 degrees C for 3 hr, was used to maximize the apoA-I immunoreactive sites in diluted serum. Serum samples extracted with chloroform-methanol, delipidated with tetramethylurea, or denatured by heating gave essentially equivalent results. The working range of the apoA-I standards was 0.5 to 5 ng and parallel responses were observed for apoA-I in serum, in isolated HDL, and in buffer as a purified apoprotein. Recovery of apoA-I added to serum was quantitative (106 +/- 3%). The intra- and interassay coefficients of variation were 6.2 and 6.9%, respectively. The enzyme immunoassay yielded values that compared favorably with those obtained by radial immunodiffusion (r = 0.84). ApoA-I concentration in African green monkey serum was highly correlated with the HDL cholesterol concentration (r = 0.86). It is concluded that this ELISA is an accurate and precise method for determination of apoA-I concentrations in primate serum.