RossouwF.T.&RowburyR.J.1984. Effects of the resistance plasmid R124 on the level of the OmpF outer membrane protein and on the response ofEscherichia colito environmental agents.Journal of Applied Bacteriology,56, 63–79.The introduction of the F‐like resistance plasmid R124 into anompCmutant ofEscherichia coliK12 conferred altered sensitivity to a wide range of inhibitory agents. Sensitivity to ampicillin, chloramphenicol, ethionine, copper ions, deoxycholate, two fatty acids and colicins L and M was decreased by the plasmid. In contrast the plasmid‐bearingompCderivatives were more sensitive than the plasmid‐freeompCmutant to erythromycin, cetyltrimethylammonium bromide and phenol. Introduction of R124 into theompCstrain also decreased the level of the OmpF protein and some (but not all) of the changed sensitivities listed above clearly resulted from this outer membrane protein deficiency. The presence in theompCmutant of R124 (rather than the more efficient introduction of the plasmid into variants of theompCstrain) led to at least most of the changes described above because those tested were accentuated by the presence of a copy mutant of R124 and reversed by plasmid