Mitochondrial monoamine oxidase isolated from bovine brain stem and purified to electrophoretic homogeneity contained 15 SH groups per mole (100000) of protein. The enzyme deaminated tyramine, p-nitro-beta-phenylethylamine, dopamine, 5-hydroxytryptamine, tryptamine but did not deaminate histamine, GABA or spermidine. Oxidation of 9-II SH groups in the MAO by air oxygen was accompanied by appearance of the properties to deaminate histamine or GABA. This qualitative alteration (transformation) in catalytic properties of the enzyme was readily reversed by treatment with reducing agents (dithiothreitol or GSH). No structural alterations detectable by electrophoresis in polyacrylamide gel were observed in course of the qualitative reversible modifications in catalytic activity of MAO. The qualitative alterations in substrate specificity were also initiated by treatment with H2O2 of the monoamine oxidases tightly bound with membrane structures of mitochondria from bovine brain stem.