ABSTRACT Three isozyme variants of catechol oxidase have been shown to be determined by alleles of a gene, Cx, which has been located on chromosome 10 less than 0.1 recombination units from the endosperm marker du 1.—The extractable form of the enzyme is modified by an endogeneous "modifier" which appears to function as an enzyme substrate. Enzyme and modifier are functionally isolated in intact cells. Modified enzyme has altered kinetics, does not migrate in electrophoresis and most probably results from a "tanning" of the enzyme by reaction products. The content of modifier varies in different lines and is genetically determined by gene(s) independent of Cx. Treatment with maleic hydrazide causes a ten-fold reduction in the modifier content of seedlings, allowing the enzyme to be extracted in an unmodified form which will migrate in electrophoresis.—This system of enzyme and modifier fits the requirements of hypersensitive disease resistance in plants and may provide a test system to investigate the biochemical basis of disease resistance.