An assay for measuring hexosaminidase A in serum and leucocytes is described in which a centrifugal analyser is used for automation of the enzyme assays after manual heat inactivation. The assay was used in a screening programme to identify heterozygotes for Tay-Sachs disease in Ashkenazi Jewish subjects in the UK. The first results from this programme indicate a carrier frequency of 1 in 27. Automation of an assay for direct measurement of hexosaminidase A in serum using 4-methyl-umbelliferyl-beta-N-acetylglucosamine-6-sulphate as substrate is also described. Comparison of data obtained from 66 control and 30 obligate carrier sera tested by this method and by heat inactivation showed improved discrimination using the sulphated substrate. Results obtained using the sulphated substrate for screening serum during pregnancy are also presented.