Several manganic porphyrins, with substituents on the methine bridge carbons, were prepared and examined for stability, redox behavior, catalysis of the dismutation of superoxide radical (O2-), and the ability to protect a superoxide dismutase (SOD)-null strain of E. coli against dissolved oxygen and a SOD-competent strain against paraquat. All of the compounds tested exhibited reversible redox behavior and were stable to EDTA in both the oxidized and reduced states, and several were able to catalyze the dismutation of O2- with the rate constants of approximately 10(7) M-1 s-1. The marked protective effects of some of these compounds exceeded that which could be anticipated on the basis of such rate constants. The tetrakis (1-methyl-4-pyridyl) compound was reduced enzymatically at the expense of NADPH and nonenzymatically by GSH and was kept in the reduced state within E. coli. Since the rate constant for reoxidation of the reduced form by O2- is 4 x 10(9) M-1 s-1, it appears that this compound acts in vivo as an NADPH/GSH:O2- oxidoreductase rather than as an SOD mimic. Its ability to facilitate aerobic growth of the SOD-null strain can be explained on this basis.