A series of man-Chinese hamster hybrids were investigated with the use of an anti-Chinese hamster hexosaminidase serum, a specific anti-human hex A serum and an anti-human hex B serum. The expression of human hex A was found to be dependent on the presence of hex B. A heteropolymeric molecule is formed independently of hex B, which consists of Chinese hamster and specific hex A moieties. It has an electrophoretic mobility nearly identical to hex A. A relationship between the absence and presence of the heteropolymeric molecule, mannosephosphate isomerase (MPI), and pyruvate kinase (PK-3), assigned to chromosome 15, was established. With respect to the two locus subunit model, the gene coding for the alpha subunit, specific for hex A, has been localized on chromosome 15.