Two hexosaminidase components, separable by starch-gel electrophoresis and possessing both beta-D-N-acetylglucosaminidase and beta-D-N-acetylgalactosaminidase activity, are present in human tissues. One of these, hexosaminidase component A, is absent in brain, liver, kidney, skin, cultured skin fibroblasts, blood plasma, and leukocytes from nine patients with Tay-Sachs disease. Hexosaminidase assay may facilitate the early diagnosis of individuals homozygous for Tay-Sachs disease.