Purified urinary alpha-N-acetylglucosaminidase acts as an exoglycosidase. The enzyme removes from heparan sulfate exclusively alpha-glycosidically linked N-acetylglucosamine residues. The pH optimum of around 4.4 towards heparan sulfate and heparin is similar to that towards synthetic arylglycosides. Urinary alpha-N-acetylglucosaminidase can be separated by isoelectric focusing into multiple forms with pI values between 3.3 and 6.0. The multiple forms differ in their recognition and endocytosis by cultivated skin fibroblasts. Forms with pI values of 4.8 +/- 0.3 respond best to endcytosis. From these forms up to 0.8 X 10(6) molecules may be recognized and taken up in an hour by a single cell. Sodium periodate treatment reduces the alpha-N-acetylglucosaminidase recognition by fibroblasts and suggests that the recognition sites on the enzyme are associated with its carbohydrate moiety. Attempts to modify the recognition of alpha-N-acetylglucosaminidase by pretreatment with purified glycosidases failed.