A new spectrophotometric method for the estimation of amine oxidase activity in beef serum was established using spermidine or spermine as a substrate of the oxidase. This method is based on the measurement of optical density at 264mμ of thiosemicarbazones of aldehydes derived from respective polyamines. From the values of molar extinction coefficients of the thiosemicarbazones, the amine oxidase activity can be expressed in terms of μmoles of spermidine or spermine oxidized per unit time and mg. protein. It was also found that one mole of spermidine and spermine were converted into one and two moles, respectively, of simple aldehyde by the enzymatic oxidation.