Maltase (α-1,4-glucosidase) activity was determined at various pH values in tissues in infantile (heart, liver, skeletal muscle), childhood (heart, liver, skeletal muscle), and adult (liver, skeletal muscle, leukocytes) types of acid maltase deficiency (AMD) and in control tissues. In an infantile case of AMD, only traces of neutral maltase were present in heart, and this activity was significantly decreased in skeletal muscle and liver. In a childhood case of AMD, neutral maltase activity was decreased only in liver. No decreases of neutral maltase occurred in adult AMD tissues. An absolute decrease of leukocyte acid maltase was found in four and a relative decrease in one of five adults with AMD. A decrease in the pH 4.0/pH 6.5 ratio of leukocyte maltase activity may aid in the diagnosis of adult AMD.