Abstract: A method for detecting a tetracycline efflux pump inhibitor in the presence of tetracycline using a reporter gene system where the tetA promoter directs transcription of a reporter gene (lacZ) while the tetA is under the control of the tet repressor encoded by the tetR gene is described. The method uses a cell having a reporter gene system where the tetA promoter directs transcription of a reporter gene (lacZ) and an active efflux system in which relatively modest levels of the efflux protein encoded by the tetA gene are produced in a constitutive manner, i.e., not under the control of the tet repressor encoded by the tetR gene. Test samples which are inhibitors of the TetA efflux protein will allow accumulation of tetracycline inside the cells at levels which will induce expression of the tetA-lacZ transcriptional fusion to give a positive signal.