In this study, 52 high risk patients who underwent operations upon the biliary tract were assigned to receive either antibiotic prophylaxis or no treatment with antibiotics. Twenty-seven patients were given 2 grams of cefamandole intramuscularly 30 minutes before operation and 2 grams every eight hours for two days postoperatively. The remaining patients were in the control group and did not receive antibiotics. Surgical wounds were inspected daily by a surgeon while the patients were in the hospital and a follow-up revision was done four weeks after discharge from the hospital. Samples of exudate or pus were taken when the wound appeared infected and cultures of aerobic and anaerobic organism done. Chi-square affinity test with Yate's correction was used for statistical results; only p values more than or equal to 0.5 were considered significant. Seven patients (28 per cent) in the control group had complications develop postoperatively; seven surgical wound infections, one of which included a subphrenic abscess. Postoperatively, there were no septic complications in the group who received cefamandole as a prophylaxis. The incidence of infection was higher for females than males. The organisms most frequently isolated were Escherichia coli and Klebsiella; only in one instance was Clostridum sporogenes found. Polymicrobial infections accounted for 42.8 per cent of the infections. No incidences were reported with the use of cefamandole in those patients who were treated prophylactically. In view of these results, we believe that cefamandole is an ideal antibiotic to be used in the prophylactic treatment of infections of high risk patients who undergo operations upon the biliary tract.