Previous studies have demonstrated that administered antibiotics must be active against major anticipated pathogens and must have reached sufficient concentrations in the tissue or body fluid at risk by the time of bacterial challenge if prophylactic therapy is to be maximally effective in reducing the infection rate of potentially contaminated surgery. The need for continuing antibiotic prophylaxis beyond the day of operation, however, has been uncertain. In a prospective, randomized, double-blind study of 220 patients undergoing elective gastric, biliary or colonic surgery, perioperative administration of cefamandole plus five days of placebo was compared to perioperative plus five days of postoperative antibiotic therapy; no significant difference was found between the groups in the rate of infection of wound (6 and 5%, respectively), peritoneum (2% each) and elsewhere (6% and 5%). In another prospective, randomized, nonblind study of 451 determinant cases of 1,624 patients undergoing emergency laparotomy, cephalothin was instituted preoper-atively but after peritoneal contamination had occurred (i.e., abdominal trauma, etc.); continued postoperative antibiotic again failed to reduce further the wound and peritoneal infection rates, as noted on comparing perioperative therapy alone (infection rates 8 and 4%, respectively) with perioperative plus 5–7 days of postoperative treatment (10% and 5%, respectively). Analysis of these data, as well as of the extra expenses incurred by 463 patients because of infection in a previous prophylactic antibiotic study, revealed an average additional expenditure of $2,686.00 for each instance of postoperative infection of the wound and/or peritoneum; whereas savings of $300.00 per patient at risk were obtained whenever appropriate prophylactic antibiotic had been given.