Measurement of hexosaminidase A (Hex A) is an important clinical chemical procedure in the classification of GM2 gangliosidosis genotypes. We have synthesized a new substrate which may be useful in both the biochemical diagnosis of GM2 gangliosidosis and the detection of heterozygotes for the Tay-Sachs disease (TSD) allele. 4-Methylumbelliferyl-beta-D-N-acetylglucosamine-6-sulfate (4MUGS) was synthesized by sulfation of 4MU-beta-D-N-acetylglucosamine (4MUG) with chlorosulfonic acid and purified through gel filtration and ion-exchange chromatography. The structure of 4MUGS was verified by elemental analysis and NMR. Hex A is approximately 100 times more active toward 4MUGS than Hex B. The advantage of this increased specificity is that Hex A can be determined in a one-step procedure which allows separation of normal control serum values from those of obligate heterozygotes. Alternatively, assay values obtained using both substrates can be transformed by application of an empirical equation that allows the calculation of both Hex A and Hex B without the requirement of thermal fractionation. Lower values for % Hex A in serum have been obtained for Tay-Sachs homozygotes using the 4MUGS assay procedure. The results of Hex A assays on fibroblast cell strains obtained from Tay-Sachs homozygotes, variant forms of GM2 gangliosidosis and normal controls are also discussed.