We studied the residual alpha-N-acetylglucosaminidase activity in two siblings with severe and mild Sanfilippo B syndrome. No striking differences were demonstrated between the mutant enzymes from the severe and the mild case. However we found an altered enzyme activity characterized by displacement of the pH optimum towards basic values compared to the pH optimum of the normal enzyme, higher stability to heat and to Hg2+ ion treatment. It is suggested that the Sanfilippo B disease in this sibship is due to a mutation of a structural gene coding for alpha-N-acetylglucosaminidase.