A rapid and sensitive colorimetric assay for cytokinin oxidase (CKO) was developed based on the copper amine oxidase property of the enzyme from cucumber cotyledons. The assay involved oxidation of isopentenyl adenine by CKO in the presence of phenazine methosulphate as an electron acceptor, resulting in reduction of nitroblue tetrazolium to formazon. The formation of formazon was linear with time for at least 20 min and could be followed spectrophotometrically at A490. CKO could also be specifically stained on polyacrylamide gels using the same principle.