Demetalation of Fe(III) microperoxidase-8 (MP8) by anhydrous HF gives metal-free MP8. a convenient starting material for a wide variety of metal-substituted MP8 derivatives, including Mn(III)MP8. Mn(III)MP8 was produced by treatment of metal-free MP8 with manganous acetate in aerated aqueous solution; it was characterized by mass spectrometry and UV-visible absorption spectroscopy. Resonance Raman spectra suggest that Mn(III)MP8 contains histidine and water axial ligands at neutral pH. The Mn(IV)=O derivative is readily prepared by oxidation of Mn(III))MP8 with hydrogen peroxide or Ru(bpy)3(3+) (bpy = 2,2'-bipyridine).