We have previously described that the quorum-sensing autoinducer, N-butyryl-L-homoserine lactone (C4-HSL) enhances mexAB-oprM expression, and this C4-HSL-mediated enhancement of mexAB-oprM expression was repressed by MexT, a positive regulator of the mexEF-oprN operon. In this study, we investigated the interaction between C4-HSL and mexR by using a knockout mutant. It was indicated that the C4-HSL-mediated enhancement of mexAB-oprM expression occurred without MexR-mediated regulation. Furthermore, it was observed that the C4-HSL-mediated enhancement of mexAB-oprM expression without the MexR-mediated regulation was repressed by MexT.