A simple radiochemical method is described for the assay of monoamine oxidase utilizing tritiated tyramine. The synthesis of tyramine-1,2-3H from tyrosine-2,3-3H is reported. Tritium, released by the oxidation of tyramine-1,2-3H by monoamine oxidase, was counted by conventional liquid scintillation techniques. Also reported is the application of this method for the study of a number of parameters which influence the activity of the enzyme, including pH, substrate concentration, and amount of enzyme. © 1969.