Pseudomonas aeruginosa and Burkholderia cepacia are the major pathogens that colonize the airway surface and cause progressive respiratory failure and high mortality, especially in cystic fibrosis (CF) patients. Tobramycin is the treatment of choice, but persistent usage enables the infectious organisms to activate defence mechanisms, making eradication rarely successful. Combinations of antibiotic and nonantibiotic compounds have been tested in vitro against P. aeruginosa and B. cepacia, but with mixed results. Sodium ions interfere with the bacterial tobramycin uptake system, but amiloride partially reverses this antagonism. In this pilot study, we extend previous findings of the effectiveness of tobramycin in combination with amiloride and other nonantibiotics against a P. aeruginosa type strain, and against four P. aeruginosa strains and one Burkholderia cenocepacia strain isolated from CF patients. Significantly, the four clinical P. aeruginosa strains were tobramycin resistant. We also find that Na+ and K+, but not Cl(-), are the chief antagonists of tobramycin efficacy. These results suggest that chemotherapy for CF patients might not only be compromised by antibiotic-resistant pathogens alone, but by a lack of penetration of antibiotics caused either by bacterial biofilms or the high sodium flux in the CF lung, or by antagonistic effects of some drug combinations, any of which could allow the persistence of drug-susceptible bacteria.