Cryptogein is a proteinaceous elicitor of plant defense (elicitin), secreted by the fungus Phytophthora cryptogea . Applied on the cut of a decapitated tobacco plant, this protein induces a hypersensitivity reaction associated to the development of a systemic resistance against pathogens. When tobacco cells were treated with this elicitin, a transient production of H₂O₂ was observed. Evidence is presented that at least a part of this production is extracellular: using cytochrome C oxido-reduction properties, we demonstrate that cryptogein elicits an extracellular production of O_{2 -.} , which is dismutated by SOD into H₂O₂. We further show that both O_{2 -.} and H₂O₂ productions are triggered by the protein phosphatase inhibitor Calyculin A and that these productions are quite identical to those elicited by cryptogein. In addition, DPI blocks the O_{2 -.} and H₂O₂ productions induced either by cryptogein or Calyculin A, suggesting that both compounds are able to activate a plasma membrane-located NADPH oxidase. Finally, a hypothetical scheme of the different elements that transduce the cryptogein-dependent signal leading to an extracellular oxidative burst is presented.