The gram-negative soil bacillus Burkholderia pseudomallei is the causative agent of melioidosis, a severe and potentially fatal septicemic disease that is endemic to Southeast Asia and northern Australia. Its intrinsic resistance to many antibiotics is attributed mainly to the presence of several drug efflux pumps, and therefore, inhibitors of such pumps are expected to restore the activities of many clinically important antimicrobial agents that are the substrates of these pumps. The phenothiazine antipsychotic and antihistaminic drugs prochlorperazine, chlorpromazine, and promazine have a synergistic interaction with a wide spectrum of antimicrobial agents, thereby enhancing their antimicrobial potency against B. pseudomallei. Antimicrobial agents that interacted synergistically with the phenothiazines include streptomycin, erythromycin, oleandomycin, spectinomycin, levofloxacin, azithromycin, and amoxicillin-clavulanic acid. The MICs of these antibiotics were reduced as much as 8,000-fold in the presence of the phenothiazines. Antimicrobial agents which did not interact synergistically with the phenothiazines include gentamicin, amoxicillin, and ampicillin. Omeprazole, a proton pump inhibitor, provided an augmentation of antimicrobial activities similar to that of the phenothiazines, suggesting that the phenothiazines might have interfered with the proton gradient at the inner membrane. B. pseudomallei cells accumulated more erythromycin in the presence of the phenothiazines, an effect similar to that of carbonyl cyanide m-chlorophenylhydrazone, a proton gradient uncoupler. In the presence of the phenothiazines, a much reduced concentration of erythromycin (0.06x MIC) also protected human lung epithelial cells and macrophage cells from B. pseudomallei infection and attenuated its cytotoxicity.