We found that the UV spectrum of benzaldehyde semicarbazone shows a maximum at 280 nm instead of at 250 nm as shown by free aldehyde. The corresponding E(280) amounts to 21,000 instead of 13,000 at E(250) On this basis, we modified the spectrophotometric method of monoamine oxidase (MAO) estimation of Tabor and his colleagues. We measured the absorbancy increment at 280 nm after stopping the MAO catalyzed reaction with 0.77 mol/1 HC10(4), with 0.1 mol/1 semicarbazide hydrochloride (SC) added. Both time of incubation (1 h) and substrate concentration (3.33 x 10^-2 mol/1) were much greater than in the original method. SC inhibits MAO and it was omitted in the incubation medium. The method offers much greater sensitivity and can be used on large series of samples with homogenates and subcellular fractions as the source of enzyme.