Antibodies have been elicited in rabbits against homogeneous ascorbate oxidase from Cucurbita pepo medullosa . These antibodies were found to inhibit the enzyme activity in a non-competitive way. They reacted faster with copper-free ascorbate oxidase but more slowly with a deglycosylated enzyme sample. Homogenates from some other plants show cross-reaction with these antibodies. The same homogenates had also an ascorbate oxidase activity roughly comparable with the extent of immunological cross-reactivity.