To understand the relationship between HLA phenotype and plasma or platelet HLA better, concentrations of plasma and platelet HLA were measured in 215 individuals of known HLA phenotypes. Precise quantitation of HLA antigens was achieved by means of an enzyme-linked immunoassay using the W6/32 monoclonal antibody and purified HLA molecules. The mean plasma and platelet HLA concentrations were 2.04 +/- 1.67 micrograms/ml (+/- SD, n = 215) and 11.28 +/- 4.65 fg/cell (+/- SD, n = 213), respectively. Statistical analysis of associations between HLA phenotypes and plasma HLA revealed that the mean plasma HLA concentration of individuals with HLA-A23 or HLA-A24 was 1.4 (p less than 0.002) or 1.9 (p less than 0.001) times higher than those without these two HLA antigens. Furthermore, the mean plasma HLA concentration of individuals who have HLA-A26 was 25% less than those without HLA-A26 (p less than 0.05). In contrast, the only association between HLA phenotypes and HLA concentrations of platelets was observed in HLA-B7-positive individuals. The mean platelet HLA concentration of HLA-B7 individuals was 27% higher than those without HLA-B7 (p less than 0.005). This finding is in accordance with previous observations made on red blood cells. The results indicate that the HLA concentrations in plasma are regulated, at least in part, by genetic factors that are different from those regulating platelet HLA.