dMucopolysaccharidosis type IIIB (MPS-IIIB) Sanfilippo type B Syndrome) is a heterosomal, recessive lysosomal storage disorder resulting from a deficiency of [alpha]-N-acetylglucosaminidase (NAGLU), To characterize this enzyme further and evaluate its potential for enzyme replacement studies we expressed the NAGLU-encoding cDNA in Chinese hamster ovary cells (CHO-K1 cells) and purified the recombinant enzyme from the medium of stably transfected cells by a two-step affinity chromatography. Two isoforms of recombinant NAGLU with apparent molecular weights of 89 and 79 kDa were purified and shown to differ in their glycosylation pattern, The catalytic parameters of both forms of the recombinant enzyme were indistinguishable from each other and similar to those of NAGLU purified from various tissues, However, compared to other recombinant lysosomal enzymes expressed from CHO-K1 cells, the mannose-6-phosphate receptor mediated uptake of the secreted form of recombinant NAGLU into cultured skin fibroblasts was considerably reduced. A small amount of phosphorylated NAGLU present in purified enzyme preparations was shown to be endocytosed by MPS-IIIB fibroblasts via the mannose-g-phosphate receptor-mediated pathway and transported to the lysosomes, where they corrected the storage phenotype, Direct metabolic labeling experiments with Na-2 (PO4)-P-32 confirmed that the specific phosphorylation of recombinant NAGLU secreted from transfected CHO cells is significantly lower when compared with a control lysosomal enzyme. These results suggest that the use of secreted NAGLU in future enzyme and gene replacement therapy protocols will be severely limited due to its small degree of mannose-6-phosphorylation.