Bovine serum amine oxidase (BSAO), reduced by excess amine under limited turnover conditions, was over 80% inactivated by H2O2 upon oxygen exhaustion. The UV-Vis spectrum and the reduced reactivity with carbonyl reagents showed that the cofactor topaquinone (TPQ) was stabilized in reduced form. The protein large M-r (170 kDa) prevented the identification of modified residues by amino acid analyses. Minor changes of the Cu2+ EPR signal and the formation of a radical at g = 2.001, with intensity a few percent of that of the Cu2+ signal, unaffected by a temperature increase, suggest that Cu2+-bound histidines were not oxidized and the radical was not the Cu+-semi-quinolamine in equilibrium with Cu2+-aminoquinol. It may derive from the modification of a conserved residue in proximity of the active site, possibly the tyrosine at hydrogen-bonding distance of TPQ C-4 ionized hydroxyl, The inactivation reaction appears to be a general feature of copper-containing amine oxidases, It may be part of an autoregulatory process in vivo, possibly relevant to cell adhesion and redox signaling.