The prophylactic use of antibiotics after "clean" operative procedures has been regarded by many physicians as unnecessary and actually harmful; they believe that such a practice aids in the development of resistant bacterial strains and are convinced that it does not reduce the incidence of postoperative infection. For many years the authors had also concurred in this opinion.1 In describing the cases of 108 patients subjected to operations using extracorporeal circulation, Kittle and Reed2 reported that the incidence of infection was as great in 46 patients who received prophylactic antibiotics (21%) as in 62 patients who did not (22%). However, Hook and Kaye3 in reviewing the treatment of bacterial endocarditis recommended the prophylactic use of an antimicrobial agent for three to five days after a cardiac operation. We have followed this policy of prophylactic antibiotic therapy for all patients in a series of 300 consecutive open-heart operations