Mitochondrial monoamine oxidase (MAO) from beef brain was shown to crossreact with antibody to purified liver MAO. Precipitin lines of identity formed between MAO prepared from the two tissues on immunodiffusion plates. It was possible to precipitate immunologically approximately 80% of the MAO activity from partially purified brain and liver extracts using anti-liver MAO. Absorption spectra of brain and liver enzymes, obtained as the difference spectra of the enzyme solution and the same solution in which enzyme had been removed by immunoprecipitation possessed essentially the same absorption maxima previously reported for purified liver MAO. All the findings indicate that the bulk of brain mitochondrial MAO is identical to liver mitochondrial MAO.