The determination of hexosaminidases A and B in most programmes for Tay-Sachs disease carrier detection is based on their different heat sensitivity (hexosaminidase A is the heat labile isoenzyme). This routine cannot be employed for individuals who also possess a thermolabile hexosaminidase B. In Israel, 0.6% of the screened samples have a labile hexosaminidases B (about 110 each year) and the assessment of their hexosaminidase A activity has hitherto been based on isoenzyme separation by ion exchange chromatography. The latter requires relative large serum samples, and the individuals must usually be reappointed. In order to avoid the thermal treatment we have used the alternative technique, which employs two substrates with different specificities for the two isoenzymes: 1. The fluorogenic substance, 4-methylumbelliferyl-N-acetyl-glucopyranoside, which measures total hexosaminidase activity and 2. the derivative, 4-methylumbelliferyl-N-acetyl glucosamine-6-sulphate, which is considerably more specific toward hexosaminidase A. Hexosaminidase A activity was expressed as a ratio of total activities (the ratio of the assay with 4-methylumbelliferyl-N-acetyl glucosamine-6-sulphate to that with 4-methyllumbelliferyl-N-acetyl-glucopyranoside). Using the results from 65 obligate heterozygotes for Tay-Sachs disease, we established our reference ranges for assigning the genotypes with respect to the Tay-Sachs gene. Comparison of the results from 182 unrelated and randomly chosen sera screened by the ratio method and by heat inactivation, showed a very high correlation (r = 0.996). Sixty eight sera with thermolabile hexosaminidase B were tested by ion exchange chromatography and by the double substrate method, and they yielded identical diagnoses with regard to the Tay-Sachs locus.(ABSTRACT TRUNCATED AT 250 WORDS)