Monoamine oxidase has been isolated and purified from bovine thyroid microsomes. The general characteristics and steady-state kinetic behavior of the microsomal enzyme have been compared with those of the enzyme isolated from bovine thyroid mitochondria. The enzymes from the two sources exhibit a high degree of substrate specificity with respect to the amines oxidized. 3-Iodotyramine is a noncompetitive inhibitor of tyramine oxidation in the case of both the mitochondrial and microsomal enzymes. Product inhibition studies suggest that the enzymes from mitochondria and microsomes catalyze reactions which proceed by a similar pathway. In contrast to the mitochondrial enzyme, the enzyme isolated from microsomes is susceptible to inhibition by anions in the following order;.