Residual acid maltase activity was found by a sensitive fluorometric assay in muscle biopsies from 15 patients with late-onset acid maltase deficiency (mean, 6.91 percent; range, 2.4 to 12.2) but not in biopsy or autopsy muscle from three patients with the infantile form. Electrophoresis, kinetic characteristics, and subcellular fractionation indicated that the residual activity was lysosomal acid maltase and not a contaminating isozyme of neutral maltase. There was no correlation between the amount of residual acid maltase activity and the severity of the clinical picture or glycogen accumulation. The presence of acid maltase activity in muscle, liver, and, to a greater extent, leukocytes in late-onset but not infantile acid maltase deficiency and the failure of the two disease forms to occur in the same family suggest that they are genetically distinct.

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