The diuretic amiloride has been under recent investigation as adjunctive therapy for pulmonary disease in children with cystic fibrosis (CF). In preliminary studies, the antimicrobial activity of this agent alone or combined with beta-lactam agents against reference strains of Pseudomonas aeruginosa and P. cepacia was poor; however, amiloride was markedly synergistic with tobramycin against P. cepacia. The purpose of this study was to determine the extent of amiloride-tobramycin synergy against CF respiratory isolates of P. aeruginosa, P. cepacia, and P. maltophilia. The MICs of tobramycin and amiloride alone against the Pseudomonas test strains were determined by agar dilution. Synergy was determined by combining each of four subinhibitory concentrations of amiloride (at least fourfold below the MIC) with doubling dilutions of tobramycin and comparing the MIC of tobramycin alone and in combination for each strain. At the highest concentration tested, the drug combination synergistically inhibited 50% of the P. cepacia strains tested; the combination was synergistic against fewer isolates of P. aeruginosa and P. maltophilia. Only P. cepacia was inhibited by tobramycin combined with amiloride at achievable airway concentrations. We conclude that the combination of tobramycin and amiloride may be potentially useful in the treatment of P. cepacia infections in children with CF.