The effect of Mn(III)-salophen, a superoxide scavenger, against oxidative stress was evaluated in <i>Escherichia coli sodA- sodB-</i>. Oxidative stress was imposed by exposure of the cells to paraquat or hyperoxia. Cells were grown in LB medium overnight, washed and resuspended in the indicated glucose/salts medium supplemented with casamino acids. The effect of Mn(III)-salophen in the oxidative stress model <i>in vivo</i> was measured in terms of the cell growth. Mn(III)-salophen ( 60 nM) completely protected <i>E. coli</i>JI132<i>sodA- sodB-</i>against 1.0 μM paraquat. Equivalent amounts of Mn(III) acetate, a Mn(III)-salophen component, also protected against paraquat toxicity in aerobic <i>E. coli</i> JI132<i>sodA- sodB-</i>. Fe(III)-salophen which has no superoxide scavenging activity, did not protect the cells against paraquat toxicity. The protective effect of Mn(III)-salophen against the paraquat toxicity was proposed to come from the intracellular superoxide scavenging activity of either the complex itself, its component Mn(III), or both, but not by inhibiting the uptake of paraquat. The protective effect of Mn(III)-salophen and Mn(III) in the glucose/salts medium containing casamino acids was also observed in <i>E. coli sodA- sodB-</i> in 100% and 50% oxygen. Hyperoxia increases intracellular levels of superoxide radicals that are intercepted by Mn(III)salophen and Mn(III). / Master of Science