Three distinct molecular subsets with different structures and alloantigenic determinants were identified in human Ia antigens from cells of an HLA-Dw7 homozygous cell line. The subsets carried DR7 specificity, BR4X7 supertypic specificity and MB2 supertypic specificity, respectively, and were immunospecifically separated by the use of operationally monospecific alloantisera. These specificities showed HLA-linked segregation in families and they were distributed in the population according to different but partially overlapping patterns. On peptide mapping analysis, the three subsets showed marked differences in the beta-chains. The alpha-chains of DR7 and BR4X7 subsets were very similar to each other, whereas the alpha-chains of MB2 subset were distinctive from those of DR7 and BR4X7. These data indicate the presence of a minimum of three HLA-linked loci; DR locus, a locus that encodes BR4X7, and a locus that encodes MB2, and substantiate the three-loci concept for the genetic control of human I antigens.