Biliary sepsis represents a major percentage of fatal complications after endoscopic retrograde cholangiopancreatography. We performed a randomized controlled study to investigate the value of antibiotic prophylaxis, and to assess the frequency and source of infectious complications associated with ERCP. Ninety-six patients who underwent 100 endoscopic retrograde cholangiopancreatographies were included in the study. Half of the patients received antibiotic prophylaxis (Cefotaxime 2 g i.v. 15 min before the procedure). Bacteremia was detected in 2% of the patients receiving antibiotic prophylaxis, as compared with 16% (p less than 0.02) in the control group. In order to determine the source of bacteremia, bile samples and irrigation fluid from the suction channel of the endo-scope were obtained for bacteriological evaluation. Several lines of evidence suggested that bacteremia associated with ERCP was essentially caused by mucosal lesions of the oropharynx. Bacteremia was asymptomatic, with the exception of two patients who subsequently developed fever, but recovered rapidly under antibiotic therapy. The frequency of cholangitis following ERCP was not significantly reduced by antibiotic prophylaxis (4% vs. 2%). Recommendations for antibiotic prophylaxis are discussed.