PRECISE localization of gallstones within the common duct remains a technical challenge despite numerous techniques devised for their easy identification. Approximately 10% of patients coming to surgery with cholelithiasis have stones within the common duct1,2 and at least 4% are reported3 to have retained stones even after the most thorough exploration. The value of operative cholangiography is undisputed, but even with this aid and meticulous operative technique, elusive stones may lie undetected within the liver, or hidden in diverticula along the common duct. This is a study of two methods for localizing common duct stones. The first utilizing radioactive labeled dye which adheres to these stones; the second by ultrasound (sonar). Materials and Methods Part I: Radioisotope Labeling. —Administration of cholecystographic dyes for each of three days prior to radiologic examination has been shown to form an opaque concentration of dye as a "rim" or "halo" on the