Many strains of Pseudomonas aeruginosa are resistant to the antibiotics cerulenin and thiolactomycin, potent inhibitors of bacterial fatty acid biosynthesis. A novel yeast Flp recombinase-based technique was used to isolate an unmarked mexAB-oprM deletion encoding an efflux system mediating resistance to multiple antibiotics in P. aeruginosa. The experiments showed that the MexAB-OprM system is responsible for the intrinsic resistance of this bacterium to cerulenin and thiolactomycin. Whereas thiolactomycin was not a substrate of the MexCD-OprJ pump expressed in a delta(mexAB-oprM) nfxB mutant, cerulenin was efficiently effluxed by the MexCD-OprJ system. It was also found that the MexAB-OprM system is capable of efflux of irgasan, a broad-spectrum antimicrobial compound used in media selective for Pseudomonas.