An electrophoretically detectable variant of acid α-glucosidase has been found in SM/J mice. This variant is attributable to excess sialylation of the enzyme and is determined by a gene, alpha-glucosidase processing, Aglp, on chromosome 17. In addition, as also reported by Potier, Lu Shun Yan & Womack (1979), SM/J mice are relatively deficient in neuraminidase and it appears that the low level of this enzyme in SM/J is determined by an autosomal codominant gene, neuraminidase-1, Neu-1. Preliminary data indicate that Neu-1 is also on chromosome 17. It seems probable that the several processing genes Apl, Aglp and Map-2 which are all closely linked on chromosome 17 are one and the same, a gene Neu-1 coding for neuraminidase.