The broad-range phytopathogenic fungus Botrytis cinerea secretes hundreds of proteins during infection of its plant hosts. One of these proteins， BcIEB1， is abundantly secreted and is able to elicit plant defenses， probably as a pathogen-associated molecular pattern， although its native function in B. cinerea biology remains unknown. Pull-down experiments designed to isolate the molecular target of BcIEB1 in tobacco resulted in the identification of osmotin， a pathogenesis-related protein of family 5 that shows antifungal activity. The expression of osmotin in Escherichia coli allowed the verification of the BcIEB1-osmotin interaction with pure proteins by pull-down and far Western blot experiments， as well as the confirmation of the activity of osmotin against B. cinerea. Interestingly， B. cinerea Δbcieb1 mutants are more susceptible than the wild-type to osmotin， and the external addition of pure BcIEB1 protects the Δbcieb1 mutants， as well as Saccharomyces cerevisiae， from the antifungal action of osmotin， thus pointing at PR5 inhibition as the primary native function of BcIEB1. The question of whether osmotin is also involved in the activation of plant defenses by BcIEB1 is also addressed， and the data suggest that osmotin does not participate in the elicitation process.