Oxidative damage to mitochondrial functions was investigated upon non-lethal treatment with H2O2 of Friend's erythroleukemia cells induced to differentiate, in comparison with the parental cell line. Both respiration and maximal ATP synthase capacity were more severely diminished by H2O2 in induced cells. The effects were mediated by intracellular redox-active iron and OH. radicals. Specifically, the mechanisms of the selective oxidant injury to F0F1 ATP synthase observed in differentiating cells likely involved impairment of F0-F1 coupling sensitive to oligomycin. We suggest a Fenton-like reaction of H2O2 with iron ions, more available in the differentiating cells, as occurring at the surface and/or in the lipid bulk phase of the inner mitochondrial membrane, thus injuring subunits responsible for the coupling of F0F1 ATP synthase through generation in situ of the actual damaging species. Besides, we propose heme iron as the most likely candidate for such reaction in induced cells actively synthesizing heme. In accordance, pretreatment of uninduced cells with hemin made H2O2-damage qualitatively identical.