To the Editor: Schimpff et al. (N Engl J Med 284:1061–1065) report that all isolates of Pseudomonas neruginosa were sensitive to carbenicillin, but Huang et al., in their study entitled “Carbenicillin in Patients with Cystic Fibrosis: Clinical pharmacology and therapeutic evaluation.” (J Pediatr 78: 338–345, 1971), showed that nearly 60 per cent of Ps. aeruginosa isolates were resistant to carbenicillin. This discrepancy is perplexing. In both studies, carbenicillin, either singly or in combination with gentamicin, was unsuccessful in the treatment of infections with Klebsietta pneumoniae or Escherichia coli. In addition, since gentamicin is ordinarily administered intramuscularly, it would be. © 1971, Massachusetts Medical Society. All rights reserved.