The bacteria found in the bile and blood of 45 patients (27 men, 18 women; mean age 69 years) suffering from acute cholangitis with gallstones were studied in order to determine the appropriate antibiotic therapy. Before any antibiotic was prescribed, a series of haemocultures was performed under aerobic and anaerobic conditions, and later gallbladder bile samples were taken during the operation. Blood cultures were positive in 55 percent of the cases and bile cultures in 84 percent. The bacteria most frequently encountered in blood were Escherichia coli, Klebsiella spp. and Bacteroides fragilis. The frequency of bacteria passage from bile to blood was studied and clearly showed a predominance of E. coli, Klebsiella and anaerobes. Enterococci passed into the blood stream only in very special circumstances, such as endoscopic sphincteromy or inadequate antibiotic therapy. To be effective, the antibiotic treatment should cover enterobacteriaceae, anaerobes and, if required, staphylococci. Enterococci can be ignored in most cases.