A manganic porphyrin (MnTMPyP), which catalyzed the dismutation of O2-, facilitated the aerobic growth of a sodA sodB strain of Escherichia coli and protected a superoxide dismutase (SOD)-competent parental strain against paraquat. Surprisingly, the latter effect was more complete than the former and the mimic could block the inductions of fumarase C and of glucose 6-phosphate dehydrogenase by paraquat, even though SOD could not. An explanation for these apparent paradoxes was found in the ability of MnTMPyP to inhibit the uptake of paraquat by E. coli. MnTMPyP was accumulated by E. coli until its intracellular concentration was 20-fold greater than the extracellular concentration. This happened in a glucose plus salts medium, but not from a rich LB medium. MnTMPyP was bound onto cellular macromolecules and was maintained in the reduced state within E. coli. The free form of the reduced MnTMPyP was autoxidizable, but the bound form was not. Consequently, the free form could catalyze the oxidation of ascorbate, while the bound form did not.