The HLA region has been assigned to chromosome 6 by a combination of family studies showing linkage with PGM3 and studies on somatic cell hybrids showing cosegregation of PGM3 with HLA and with chromosome 6 and its other syntenic markers ME(S) and SOD(M). The gene for β2 microglobulin (β2m), the 12,000 molecular weight polypeptide associated with the HLA A, B, and C locus products, has, on the other hand, been assigned to chromosome 15. Serological detection and definition of HLA antigens on human cells is almost always based on complement dependent cytotoxicity assays. The authors have used such an assay, coupled with absorption analysis, to demonstrate the clear cut and specific expression of HLA antigens on a series of human mouse hybrids. The results confirm directly the assignment of HLA to chromosome 6 and show that HLA A and B locus specificities are expressed independently of the presence of human β2m, namely, in hybrids containing human chromosome 6 but not 15