Assays of the N-acetyl-β-glucosaminidase activity in sera and in liver tissues of patients with hepatic disorders were carried out by the method of Walker, using p-nitrophenyl-N-acetyl-β-glucosaminide as substrate. The results were as follows: 1. The normal activity of serum N-acetyl-β-glucosaminidase was 5.7 units in average. There was no significant difference in sex. 2. Serum N-acetyl-β-glucosaminidase activity showed a distinct increase in acute hepatitis, recurrent chronic hepatitis, precirrhosis and active cirrhosis. In active and inactive chronic hepatitis and inactive cirrhosis, the activity was slightly elevated. 3. In liver diseases the activity of serum N-acetyl-β-glucosaminidase changed independently of that of serum β-glucuronidase. 4. N-acetyl-β-glucosaminidase activity in serum showed a close correlation with that in liver tissues and such histological findings as "degeneration of liver cells", "Kupffer cell activation", "cell infiltration", "proliferation of connective tissue" in the liver. 5. The elevation of serum N-acetyl-β-glucosaminidase activity has a close relation to the proliferation of connective tissue in liver diseases, while it has a close relation to the acute destructive change such as degeneration and necrosis of the hepatic cells. 6. In obstructive jaundice and hepatic carcinoma, remarkably elevated activities of serum N-acetyl-β-glucosaminidase were observed.