The a-galactosidases in normal man-Chinese hamster somatic cell hybrids were investigated with antibodies specific for human a-galactosidase A and antibodies specific for Chinese hamster a-galactosidase. It was found that an isoenzyme in hybrid cells, which has an electrophoretic mobility between that of human a-galactosidase A and Chinese hamster a-galactosidase, contains immunologic determinants of both human and Chinese hamster origin, suggesting that it is a heteropolymeric molecule. Moreover, the locus for human a-galactosidase, which was found to be X-linked, is the locus coding for a-galactosidase A. Hybrids isolated after fusion of Chinese hamster cells with cells of a patient with Fabry's disease did not express human a, galactosidase A or the heteropolymeric molecule even in the presence of the active human X chromosome, indicating that the deficiency ofa-galactosidase A in Fabry's disease is probably due to a mutation in a structural gene resulting in the inability to form immunologically detectable and functionally active molecules of a-galactosidase A.