We describe a method for continuous kinetic measurement of hexosaminidase activity, and have applied it to detection of heterozygotes for Tay-Sachs gene. In contrast to existing single-point methods, a ph of 4.5, which is optimal for hexosaminidase activity on the substrate (4-methylumbelliferyl-N-acetyl-beta-d-glucosaminide) is maintained while the increase in fluorescence produced by 4-methylumbelliferone, the reaction product, is being determined. Under the conditions described the reaction follows zero-order kinetics, and activity is linearly related to serum dilution. There is a fairly narrow but practicable range of optimal substrate concentration. The presence of substrate in concentrations exceeding 1 mmol/liter results in progressive quenching of fluorescence and a decrease in apparent enzyme activity. The procedure is standardized with 4-methylumbelliferone in a solution of the substrate. This method is adaptable to use with automated multiple-point discrete-sample devices and especially with the fluorometric kinetic analyzers now being developed.