Previous studies demonstrated that higher biliary protein is associated with reduced metastability of bile. This study attempted to examine the induced effect of ursodeoxycholate on metastability of bile by measuring the nucleation time and biliary protein in cholesterol gallstone patients. Thirty-seven patients with functioning gallbladders were studied 10 control patients without gallstones and 27 with cholesterol gallstones. Ten of 27 cholesterol gallstone patients were treated with ursodeoxycholate (600 mg/day) prior to surgery. Twelve of 17 untreated gallstone patients had cholesterol crystals in gallbladder bile while cholesterol crystals were absent in the ursodeoxycholate-treated gallstone patients and in the controls. Total protein concentration and cholesterol saturation index were significantly greater in the untreated gallstone patients with crystals than in those without crystals in bile. The treatment with ursodeoxycholate significantly decreased biliary protein concentration and cholesterol saturation index associated with the prolonged nucleation time. Cholesterol nucleation time correlated with biliary total protein concentration and cholesterol saturation index but not with total lipid concentration. It is concluded from the present study that ursodeoxycholate decreases biliary protein thereby partly increasing metastability of gallbladder bile.