Recent studies on platelet HLA indicate that greater than 50% of platelet HLA antigens are adsorbed on the platelet surface and may be derived from plasma. It has been speculated that platelet HLA may be directly proportional to plasma HLA concentration. To determine the quantitative correlation between plasma and platelet HLA, a precise competitive enzyme-linked immunoassay (ELISA) for measurements of soluble and cellular HLA antigens was developed by using purified HLA antigens and W6/32 anti-HLA monoclonal antibody. The useful range of the standard curve for the assay was 0.01 to 5.0 micrograms/mL. The intraassay and interassay variations were 7% and 14%, respectively. The plasma HLA concentrations measured in 61 healthy adults ranged from 0.25 to 4.1 micrograms/mL, and the mean plasma HLA concentration was 1.47 +/- 0.87 micrograms/mL (+/- SD). Platelet HLA concentrations determined in the same 61 persons ranged from 4.7 to 17.33 fg/platelet, and the mean concentration was 9.3 +/- 2.9 fg/platelet (+/- SD). Chloroquine-elutable platelet HLA concentrations were also determined in 42 of the 61 persons, with the mean value of 5.7 +/- 2.1 fg/platelet (+/- SD). The plasma HLA concentration of each individual was then correlated with the same person's total or chloroquine-elutable platelet HLA concentration. Linear regression analyses of the results revealed no significant correlation between platelet and plasma HLA concentrations. Thus, it is unlikely that chloroquine-elutable HLAs are derived from plasma. The developed solid-phase assay for HLA will be useful for further study of the quantitative significance of plasma HLA antigens in allosensitization of transfused individuals.