Cultured skin fibroblasts from two patients with Sanfilippo syndrome, Type B were strikingly deficient in alpha-acetylglucosaminidase activity (alpha-2-acetamido-2-deoxy-D-glucoside acetamidodeoxyglucohydrolase, EC 3.2.1.X). A similar deficiency was found in frozen organs from two other patients. A partial deficiency of alpha-acetylglucosaminidase was found in cultured skin fibroblasts from both parents of one patient. Soluble endogenous inhibitors did not account for the enzyme deficiency. Other lysosomal hydrolases were normal or increased in cultured fibroblasts from patients with this disease. No deficiency of alpha-acetylglucosaminidase is present in other genetic mucopolysaccharidoses, including Sanfilippo Type A.