Previously, we identified a soluble salicylic acid (SA)-binding protein (SABP) in tobacco whose properties suggest that it may play a role in transmitting the SA signal during plant defense responses. This SA-binding activity has been purified 250-fold by conventional chromatography and was found to copurify with a 280-kDa protein. Monoclonal antibodies capable of immunoprecipitating the SA-binding activity also immunoprecipitated the 280-kDa protein, indicating that it was responsible for binding SA. These antibodies also recognized the 280-kDa protein in immunoblots of the partially purified SABP fraction or the crude extract. However, when the crude extract was prepared in the presence of antioxidants, only a 57-kDa protein was recognized. Since the SABP has a native molecular mass of 240 kDa, it appears that the SABP is a complex which contains a 57-kDa subunit and perhaps one or more additional proteins which are covalently crosslinked in the absence of antioxidants. The ability of a variety of phenolic compounds to compete with SA for binding to the SABP was both qualitatively and quantitatively correlated with their biological activity in inducing defense-related genes. Moreover, the inducibility of the pathogenesis-related (PR)-1 genes by SA was proportional to the abundance of the SABP in different organs. These correlations are consistent with a role for the SABP in perceiving and transducing the SA signal in plant defense.