Themost efficaciousMn(III) porphyrinic (MnPs) scavengers of reactive species have positive charges close to the Mn site, whereby they afford thermodynamic and electrostatic facilitation for the reaction with negatively charged species such as O2•- and ONOO-. Those are Mn(III) meso tetrakis(N-alkylpyridinium-2-yl)porphyrins, more specificallyMnTE- 2-PyP5+ (AEOL10113) and MnTnHex-2-PyP5+ (where alkyls are ethyl and n-hexyl, respectively), and their imidazolium analog, MnTDE-2-ImP 5+ (AEOL10150, Mn(III) meso tetrakis(N,N'-diethylimidazolium-2-yl) porphyrin). The efficacy of MnPs in vivo is determined not only by the compound antioxidant potency, but also by its bioavailability. The former is greatly affected by the lipophilicity, size, structure, and overall shape of the compound. These porphyrins have the ability to both eliminate reactive oxygen species and impact the progression of oxidative stressdependent signaling events. This will effectively lead to the regulation of redox-dependent transcription factors and the suppression of secondary inflammatory- and oxidative stressmediated immune responses. We have reported on the inhibition of major transcription factors HIF-1α, AP-1, SP-1, and NF-ΚB by Mn porphyrins. While the prevailing mechanistic view of the suppression of transcription factors activation is via antioxidative action (presumably in cytosol), the prooxidative action ofMnPs in suppressing NF-ΚB activation in nucleus has been substantiated. Themagnitude of the effect is dependent upon the electrostatic (porphyrin charges) and thermodynamic factors (porphyrin redox ability). The prooxidative action of MnPs has been suggested to contribute at least in part to the in vitro anticancer action ofMnTE-2-PyP5+ in the presence of ascorbate, and in vivo when combined with chemotherapy of lymphoma. Given the remarkable therapeutic potential of metalloporphyrins, future studies are warranted to further our understanding of in vivo action/s of Mn porphyrins, particularly with respect to their subcellular distribution. © Springer-Verlag 2010.