A method for assaying a compound of the formula ##STR1## wherein R.sub.1 is hydroxyl or amino, or hydrogen if at least one of R.sub.2, R.sub.3, R.sub.4, R.sub.5 and R.sub.6 is hydroxyl or amino, and R.sub.2, R.sub.3, R.sub.4, R.sub.5 and R.sub.6 are hydrogen, halogen, lower alkyl, lower alkoxy, amino, substituted amino, hydroxy, carboxyl or sulfo, or R.sub.5 and R.sub.6 together form a ring, comprises establishing a reaction system containing the compound to be assayed and a coupler and an enzyme capable of consuming oxygen and generating a pigment in the presence of the compound and the coupler. A detectable change in the reaction system is measured, to assay the compound in question. The measurement can comprise the measurement of consumed oxygen, as by an oxygen electrode. Or the measurement can comprise the measurement of the generated pigment, as by a colorimetric assay at a specific absorption wavelength thereof.