SUMMARY: Carbenicillin, a 9-alpha carboxybenzyl penicillin, is bactericidal only for Escherichia coli strains which are resistant to less than 125 μg./ml. of ampicillin. All E. coli and Salmonella typhimurium strains in which penicillinase is a surface enzyme are resistant to Carbenicillin. E. coli and S. typhimurium strains in which production of beta-lactamase is episomally mediated are resistant to both ampicillin and Carbenicillin. A non-hydrolyzable penicillin (methicillin or dicloxacillin) does not allow Carbenicillin to exert its anti-bacterial effect against resistant strains of E. coli, Pseudomonas, Klebsiella or Enterobacter. Carbenicillin shows no synergy with penicillinase-resistant penicillins.