The microflora in bile from the gallbladder and common bile duct was investigated in 303 patients who underwent surgery for cholelithiasis. The purpose of this study was to identify current bacteria and bacterial casts in the biliary tract and also to analyze the relationship between bactericholia at the time of operation and postoperative infection. Bile cultures were positive in 38% of all patients, although a higher incidence of positive bile cultures occurred in patients over 70 years of age (77%), those with common duct stones (83%), those with pigment stones (65%), and those who underwent gastrectomy (71%). The predominant organisms were Escherichia coli (22%), Klebsiella (18%), and Enterococcus (15%). Obligate anaerobes were less frequently seen (4%), being found only in patients with pigment stones and always mixed with aerobes. Four patients developed postoperative infections (1.3%) which were all caused by biliary bacteria. The following two factors may contribute to this low incidence of postoperative infections: our policy of operating electively whenever possible, and the prophylactic use of antibiotics to which the organisms cultured from bile are sensitive.