A girl with congenital aortic stenosis underwent open commissurotomy. Postoperatively, she developed Enterobacter cloacae bacteremia followed by endocarditis. The combination of carbenicillin disodium and kanamycin sulfate was shown by in vitro tests to have a synergistic, bactericidal effect against this organism. The administration of these antibiotics resulted in cure of the endocarditis. Carbenicillin may be useful in the treatment of endocarditis due to gram-negative bacteria.