The gallbladder wall, gallbladder lumen and bile duct intraoperative sampling materials had been examined in 52 patients with an operation on the biliary tract. All patients were treated by only one infusion before operation: 1 g ceftriaxone was given to 26 patients and 2 g ceftriaxone to the 26 others. Bacterial species were isolated on twenty patients. There was no significant difference in infection rate and organism identity between bacterial flora isolated from patients receiving 1 g ceftriaxone and patients receiving 2 g. Isolated germs were enterobacteriaceae (15) streptococci (14) and anaerobic bacteria (8). The mean count of bacteria in bile is 10(4) germs/ml. Half positive sampling materials contains more than one bacterial strain. The bacterial flora isolated from bile has characteristics of mixed flora infection. Ceftriaxone had a good efficacy in antibiotic prophylaxis.