Harpin from the bean halo-blight pathogen Pseudomonas syringae pv phaseolicola (harpin(Psph)) elicits the hypersensitive response and the accumulation of pathogenesis-related gene transcripts in the nonhost plant tobacco. Here, we report the characterization of a nonproteinaceous binding site for harpin(Psph) in tobacco plasma membranes, which is assumed to mediate the activation of plant defense responses in a receptor-like manner. Binding of 125I-harpin(Psph) to tobacco microsomal membranes (dissociation constant = 425 nM) and protoplasts (dissociation constant = 380 nM) was specific, reversible, and saturable. A close correlation was found between the abilities of harpin(Psph) fragments to elicit the transcript accumulation of the pathogenesis-related tobacco gene HIN1 and to compete for binding of 125I-harpin(Psph) to its binding site. Another elicitor of the hypersensitive response and HIN1 induction in tobacco, the Phytophthora megasperma-derived beta-elicitin beta-megaspermin, failed to bind to the putative harpin(Psph) receptor. In contrast to activation by beta-megaspermin, harpin(Psph)-induced activation of the 48-kD salicylic acid-responsive mitogen-activated protein kinase (MAPK) and HIN1 transcript accumulation were independent of extracellular calcium. Moreover, use of the MAPK kinase inhibitor U0126 revealed that MAPK activity was essential for pathogenesis-related gene expression in harpin(Psph)-treated tobacco cells. Thus, a receptor-mediated MAPK-dependent signaling pathway may mediate the activation of plant defense responses induced by harpin(Psph).