MICs of ticarcillin, tobramycin, and rifampin alone and in combination were determined by a three-dimensional checkerboard method for 33 isolates of Pseudomonas aeruginosa. Twenty-nine of 33 isolates were affected synergistically (FIC index less than 1.0) by the combination of ticarcillin-tobramycin, including 13 of the 16 isolates resistant to ticarcillin and/or tobramycin alone. However, despite fulfilling the criteria for synergy, the MIC of the ticarcillin and/or tobramycin when in combination still exceeded attainable serum concentrations for 12 of the 13 resistant isolates. When rifampin was added to ticarcillin-tobramycin, a synergistic interaction was observed for all 33 isolates. Furthermore, of the 16 isolates resistant to ticarcillin and/or tobramycin, eight were inhibited by attainable concentrations of all three antibiotics in combination. If these results can be confirmed in the clinical situation, application of the three-drug combination of ticarcillin-tobramycin-rifampin might improve survival in selected patients with serious P. aeruginosa infections.