Assays of the β-glucuronidase activity in sera and in liver tissues of patients with liver diseases were carried out by the method of Masuya and Tsukamoto, using p-nitrophenyl-β-D-Glucopyranosiduronic acid as substrate. The results were as follows: 1. The mean enzyme activity in normal sera was 885 units. The average activity for men was rather higher than that for women. 2. Serum β-glucuronidase activity showed a distinct increase in acute hepatitis, compensated liver cirrhosis and hepatic carcinoma and a slight elevation in recurrent chronic hepatitis, active chronic hepatitis, precirrhosis and obstructive jaundice. On the contrary, the activity was almost normal in inactive chronic hepatitis and abnormally low in decompensated cirrhosis. 3. In acute hepatitis, serum β-glucuronidase activity was markedly high during the acute phase and returned gradually and simultaneously with transaminase to the normal level as a sign of recovery. In chronic hepatitis, precirrhosis and cirrhosis of the liver, serum β-glucuronidase activity fluctuated independently of transaminases and the period of low β-glucuronidase activities was in accordance with that of hepatic failure. A rise of the activity following this period was an early sign of recovery and a return to the normal level coincided with a further clinical improvement. 4. β-Glucuronidase in liver tissues of patients with hepatic diseases showed not only an increase but also a decrease of the activity. The activity alterations were closely related to degeneration, necrosis and regeneration of hepatic cells, while cases with severe histological changes exhibited a reduced activity. 5. There was a close correlation between the activity changes of β-glucuronidase in serum and those in liver tissues. 6. These data suggest that in liver diseases the alterations of serum β-glucuronidase activity reflect sensitively active histological changes of the liver including cellular damage and repair.