Plant cells respond to pathogen attach with a burst of H2O2 secretion. The question whether this defense reaction is catalysed by a NAD(P)H oxidase similar to the NADPH oxidase of phagocytic leukocytes in mammals or by an extracellular peroxidase is presently a matter of controversial debate. The observation that H2O2 production by plant cells can be inhibited by diphenyleneiodonium (DPI), a potent inhibitor of the mammalian NADPH oxidase, has fostered the view that a mammalian-type enzyme is responsible for the H2O2 production by plant cells. Here we show that DPI inhibits the NADH-dependent H2O2 production by horseradish peroxidase in the same concentration range as previously used for the inhibition of putative NADPH oxidase activity in plants. The peroxidative activity normally used for assaying peroxidase is not affected by DPI, indicating that the inhibitor specifically interferes with a partial reaction that is exclusively involved in the O2 reducing activity of the enzyme.