To understand the complexity of plasma HLA antigens, the distribution of different molecular weight forms of class I HLA in plasma was investigated in 44 HLA-phenotyped and unrelated individuals. Plasma class I HLA were immunoprecipitated by using the W6/32 anti-HLA monoclonal antibody, separated by SDS-polyacrylamide gel electrophoresis and characterized by immunoblotting with the HC-10 monoclonal antibody. Four different forms of HLA heavy chains (HLA-HC) with relative molecular masses of 44, 39, 36, and 34 kd were detected. Plasma samples from all individuals contained 44 and 36 kd HLA-HC, but varied as to the presence of 39 and 34 kd HLA-HC. Eighteen percent of the individuals did not have any detectable class I HLA with 39-kd heavy chains in their plasma and 61% did not have plasma class I HLA with 34-kd heavy chains. Thus, four different distribution patterns were identified for plasma class I HLA among all individuals included in our study. The distribution patterns in four different individuals were evaluated quarterly and remained unchanged during 1 year follow-up. A significant association of absence of 39-kd plasma class I HLA-HC with female gender (p less than 0.05) and HLA-B7 phenotype (p less than 0.00015) was also found. Further pedigree analyses of four families of HLA-B7-positive and 39-kd HLA-HC-negative probands indicated that genetic factor(s) other than those associated with HLA-B7 allele and female gender is involved in regulating the expression of the plasma class I HLA with 39-kd heavy chains.