Generation of the microbicidal oxidative burst in human neutrophils requires participation of four proteins, a membrane bound flavocytochrome b-558, two soluble proteins termed p47-phox and p67-phox, and the Ras-related GTPase Rac. Because plant cells exposed to pathogens produce a similar oxidative burst, we have looked for similarities between the oxidase complexes of the two systems. Antibodies against human neutrophil p47-phox and p67-phox were used to immunoblot cell extracts from several plant cell lines and were found to cross-react with proteins of the same molecular weight. Furthermore, plant cell lines not previously shown to produce an oxidative burst, yet found to express these immunoreactive proteins, rapidly generated hydrogen peroxide in response to elicitation. Finally, diphenylene iodonium (DPI) and α-naphthol, known specific inhibitors of the NADPH oxidase in neutrophils, also inhibited the oxidative burst in soybean cell suspensions with similar K₁ values (about 15 μM and 30 μM, respectively). These results provide evidence for involvement of proteins related to the neutrophil oxidase complex in the defense-related oxidative burst of plants.