Serum N-acetyl-beta-D-hexosaminidase is compared quantitatively and qualitatively in 14 obligate heterozygotes for the mutant gene causing I cell disease (ICD) or mucolipidosis II and in 31 normal controls. The average specific activity in either group is significantly different but reliable heterozygote detection cannot be achieved because of some overlapping of the ranges of individual results. Fractionation of the enzyme either by DEAE cellulose column chromatography, or by heat inactivation, yields a typical average result for each genotype. Also, mere expression of the various components as percentages of the total activity is not useful for certain identification of the ICD heterozygote. There is considerable overlapping of the percents hexosaminidase I1 and A in both groups of sera. If enzymatic hydrolysis by any component is expressed as a partial activity, a much better though not absolute distinction between the ICD heterozygote and the normal control becomes possible. Only the latter way of expression of hexosaminidase results makes distinction between the ICD heterozygote and the Tay-Sachs heterozygote very probable.