1. The N-acetyl-β-glucosaminidase of human spleen has been separated by gel electrophoresis into two components, an acidic form A and a basic form B. 2. The two forms are readily separated on DEAE-cellulose and have been concentrated 50-fold and sevenfold respectively. 3. They show similar Km values towards 4-methylumbelliferyl N-acetyl-β-d-glucosaminide, and have the same pH optima when compared in citrate, phosphate or acetate buffers. They are inhibited to a similar extent by acetate, heparin, N-acetylgalactosaminolactone, N-acetyl-β-d-galactosamine and N-acetyl-β-d-glucosamine. Specificity for C-4 orientation is not absolute and p-nitrophenyl β-galactosaminide is also hydrolysed but at a rate only 11·6% of that for the corresponding glucosaminide. 4. N-Acetyl-β-glucosaminidase B is stable over a wider pH range than is N-acetyl-β-glucosaminidase A, and is less easily denatured by heat. 5. Tissue fractionation indicates that both the A and B forms are present in the lysosomal fraction, whereas the supernatant contains the A form only. 6. Evidence is presented to indicate that the A form contains a number of sialic acid residues.

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