Mitochondrial monoamine oxidase (MAO) from bovine liver is separable into two pure components (C1 and C2) and a crude protein fraction (Fraction A). Antibody to the larger of the two pure components, (C2); was prepared in rabbits. All the separable forms of MAO were shown to cross react with the antibody to C2, forming lines of antigenic identity on double diffusion agar plates. Immunoprecipitin titrations demonstrated that all forms of MAO activity could be quantitatively precipitated using anti-C2, although the soluble antigen-antibody complexes which formed with Fraction A required precipitation with goat anti-rabbit globulin. These results indicate that all the separable forms of liver MAO are antigenically indistinguishable and that the smaller component has all the antigenic sites present on the larger form of the molecule. The antigenic data strongly support the concept that the separable forms of MAO are composed of the same protein subunit assembled or aggregated into forms of different molecular weight.