Iron, particularly in the ferrous state plays a role in regulating the biosynthesis of the manganese superoxide dismutase (MnSOD) in Escherichia coli B. Addition of iron has a repressive effect on the synthesis of MnSOD under normal or inducing conditions (i.e. in the presence of paraquat). Addition of manganese to cultures already depressed for MnSOD biosynthesis causes a further increase in the amount of active enzyme, however, this effect is also abolished by the addition of iron. Removal of metals from the growth medium by Chelex 100 also derepresses the synthesis of MnSOD but repletion of the medium with iron abolishes this effect. Chelators specific for Fe2+, 2,2'-dipyridyl, and 1,10-phenanthroline, cause a 5-7-fold increase in MnSOD. Removal of iron also increases the synthesis of MnSOD in the absence of oxygen. A model is presented to account for the observed effects of oxygen, superoxide anion, and chelators on the increased synthesis of MnSOD. In this model, the regulatory repressor for MnSOD biosynthesis is envisioned as being an iron-containing protein.