Background/Aims: The most recognized function of the human gallbladder(GB) is to store bile. However, this organ is not a static reservoir. It actively modifies bile by the double process: concentration and acidification. This study was designed to compare solute composition in GB bile between patients with and without gallstones and to evaluate the effects of concentration and acidification of GB bile on cholesterol and calcium solubility. Methods: Bile was sampled directly from the GB of fifteen patients with gallstones(gallstone group) and of fourteen patients undergoing laparotomy for reasons unrelated to hepatobiliary disease(control group). In the bile, we measured electrolytes, pH, pCO, protein, cholesterol, phospholipid, and bile salt. And then, we calculated the cholesterol saturation index(CS1) by using the critical table of Carey and the CaCO saturation index(CaCO Sl) by using metbods of Rege and Moore. Results: The pH was significantly higher in gallstone group. The bile salt concentration was significantly )ower in gallstone group. In between the two groups, total protein concentration were not significantly different. The CSI and CaCO SI were higher in gallstone group, but were not significantly higher than in control group. There was no significant correlation between pH and bile salt concentration, but a significant correlation was found between pH and CaCO Sl. There was also no significant correlation between pH and bile salt concentration, CSI and bile salt concentration, and pH and CSl, respectively. Conclusions: Failure of concentration and acidification of bile was found in patients with gallstones. Concentration and acidification of bile were independent processes. Acidification of bile prevents gallstone formation by increasing calcium salt solubility. (Korean J Gastroenterol 1995;27:338 - 344)