Human beta-galactosidase and alpha-neuraminidase deficient mucolipidosis [ML(gal-neur-)] is an inherited lysosomal enzymopathy which recently was designated as a sialidosis. We analyzed the neuraminidase deficiency of this disorder with genetic complementation analyses using a heterokaryon enrichment procedure. The genetic defects of two apparent variants of this disorder complemented the defects of the neuraminidase deficiency diseases, sialidosis I and mucolipidosis I, resulting in the restoration of neuraminidase activity in heterokaryons. The neuraminidase deficiency, therefore, may not be the primary defect in ML(gal-neur-) and is not an appropriate test for determining carrier status. The clinical and biochemical characteristics of this disorder suggest that a post-translational or processing event for these enzymes may be defective. The defect, however, is different from I-cell disease and pseudo-Hurler polydystrophy, two disorders of post-translational lysosomal enzyme biosynthesis, since complementation studies demonstrated recovery of intracellular beta-galactosidase and alpha-neuraminidase levels in heterokaryons. The lack of human beta-galactosidase expression in man-mouse somatic cell hybrids formed from fibroblasts of the infantile onset type disorder suggests that the defect is not corrected by the mouse genome. The ML(gal-neur-) disorder therefore appears to be a distinct subtype of the inherited neuraminidase deficiencies in which the defect mat occur in a post-translational or regulatory step which coordinately affects the expression of lysosomal beta-galactosidase and alpha-neuraminidase.