To evaluate image clarity in operative cholangiography with changing radiographic variables, in vitro experiments were performed in which 1-mm radiographically nonopaque biliary calculi in tubes were examined. Variables changed in these studies were tube caliber (6, 13 and 20 mm), contrast concentration (15, 30, and 60%), X-ray exposures (1, 2, 4, 8, 16, 32, 64, and 128 mAs) and location of calculi (central or peripheral). Sensitivity for the detection of calculi and the clarity of contrast radiographic images depended upon these variables. Reduced contrast concentration or increased radiographic exposure were needed for large caliber tubes. Central small calculi required more exposure than peripheral calculi. Practical inferences from these studies can be summarized as guidelines for performing operative cholangiography: (1) Surgeons should become aware of exposure settings used on the radiographic equipment in the operating room. In particular, the milliampere-seconds-setting is an important variable. (2) Normal size and slightly enlarged ducts should be injected with 30% radiographic contrast. (3) Ducts which are 20 mm or more in diameter should be injected with 15% contrast. (4) Variable radiographic exposures may be necessary to demonstrate centrally and peripherally located calculi, especially for small calculi in large ducts.