Uptake of tetracydines into Escherichia coli was assessed with a strain carrying a tetA–lacZ translational fusion, in which expression of the enzyme is controlled by the pSC101 tetR repressor gene, by examining β-galactosidase induction. The ability of tetracycline analogues to induce β-galactosidase synthesis was correlated with their hydrophobicity, such that hydrophobic analogues were poor enzyme inducers. Treatment of E. coli with polymyxin B nonapeptide (PMBN) rendered cells more permeable to minocycline, but not to tetracycline.