An outer membrane protein (OprK) overproduced in a multiply antibiotic-resistant strain of Pseudomonas aeruginosa was previously identified as the product of the third gene of a multidrug resistance operon, mexA-mexB-oprK (K. Poole, K. Krebes, C. McNally, and S. Neshat, J. Bacteriol. 175:7363-7372, 1993). To determine whether this protein was identical to another outer membrane protein (OprM) also overproduced in some multiply resistant strains, attempts were made to map the transposon insertion site of several OprM-deficient mutants to the mex operon. Amplification of chromosomal DNA of several Tn5 insertion OprM-deficient mutants with primers specific to each gene of the mex operon revealed that the transposon had inserted into mexB in one instance and into oprK in two others. Furthermore, introduction of the cloned mexA-mexB-oprK operon into these mutants restored expression of multidrug resistance, concomitant with OprM production. These data demonstrated that OprM is encoded by the mex operon. OprM and OprK were not, however, immunologically cross-reactive, indicating that they are distinct proteins and that OprK is, in fact, not encoded by the mex operon. This operon is thus renamed mexA-mexB-oprM.