Benign disease of the biliary tract has become the most common indication for major abdominal surgery. Septic complications play an increasingly important role in the morbidity and mortality of biliary tract disease and biliary surgery. This prospective study deals with 800 consecutive cholecystectomies in which bacteriology data were studied and correlated with clinical data. The general incidence of positive bile cultures was 27%. Patients with a high incidence of positive cultures (high-risk group) included those with acute cholecystitis, jaundice, choledochal stones, diabetes mellitus, nonfunctioning gallbladders, and patients over 70. Patients not presenting any of the above features (low-risk group) had a very low incidence of positive bile cultures (less than 7.1%). The types of bacteria cultured were gram-negative bacteria in the great majority of cases, gentamicin showing the highest sensitivity rate (91%). Close correlation was found between positive bile cultures and the incidence of septic complications. The results of this study support the view that perioperative antibiotic prophylaxis in biliary surgery should be given only to those patients defined as high risk. In the low-risk group, about 60% of all cases, antibiotics can be omitted safely.