Two clinically verified cases of Tay-Sachs disease and the family members of one of these cases were assayed for lysosomal enzymic activities in serum, leucocytes and skin biopsies. Among the enzymes assayed (mannosidase, galactosidase, galactosaminidase [including the A and B isoenzymes], fucosidase and acid phosphatase), the total galactosaminidase activity was found decreased in both skin biopsies and leucocytes of one of the two patients, In the other patient the isoenzyme A galactosaminidase activity was found relatively decreased in serum associated with an increase (147%) in the total serum activity. This was, however, not so in the first mentioned case, where the total serum activity was only 10% of the total serum activity of normal individuals. The assay of the lysosomal activities among family members made it possible to trace the carriers of the disease.