Bacterial survival indicates that copper or iron is an essential mediator in paraquat toxicity in Escherichia coli [Kohen, R., & Chevion, M. (1985) Free Radical Res. Commun. 1, 79-88; Korbashi, P., Kohen, R., Katzhendler, J., & Chevion, M. (1986) J. Biol. Chem. 261, 12472–12476]. In this study we have identified the cytoplasmic membrane as a target organelle in metal-mediated paraquat toxicity and have demonstrated the complete correlation of the membrane damage with the levels of adventitious copper (or iron). The extent of membrane damage was related by use of four parameters: (a) the level of cellular ATP, the level of cellular potassium, the cellular capacity to accumulate and retain radiolabeled leucine, and (d) the cellular integrity as reflected by transmission electron microscopy (TEM). Exposure of bacterial cells to a combination of paraquat and copper caused a marked decline in parameters a, b, and c. This decline was found to occur in parallel with, or even to precede, the sharp loss of survival of E. coli under the same conditions. Likewise, TEM micrographs clearly indicated alterations in cellular structure that possibly reflect sites of detachment of the cytoplasmic membrane from the bacterial capsule. In contradistinction, copper alone or paraquat alone could not bring about similar changes in cellular structure. These findings are in accord with the suggested site-specific metal-mediated Haber-Weiss mechanism for paraquat toxicity and support our notion that specific chelators of transition metals could reduce or prevent the biological deleterious effects of this herbicide.