The relationship between 7 antigenic groups of lymphocytes was studied in 36 families with 144 children and a population of 2208 people. Gene frequencies of the individual antigens were deduced from the phenotype of the subjects in the population and the parents in the families as well as the genotype of the latter. In families these antigenic determinants were inherited en bloc, confirming the concept of Dausset, van Rood, Ceppellini, Amos and others that these antigens belong to a single complex locus, theHL-Alocus. Evidence is presented to suggest that there are two mutational sites in theHL-Alocus. One site corresponds to antigens 1 and 2 (LA series of Payne-Bodmer). The second site is principally defined by the antithetic groups 5 and 6 (roughly 4d and 4c). Two other groups, 3 and 7 (approximately 4a and 4b of van Rood), which behave allelic but are more difficult to define sharply, appear to constitute a partially independent site next to the 5–6 site. The mutational site next to 5 appears to be group 7, whereas the site next to antigen 6 could be either 3 or 7. Thus theHL-Alocus seems to have two sites, of which one is composed of two partially independent subunits.