The rice Xa21 gene confers immunity to most strains of the bacterium Xanthomonas oryzae pv. oryzae (Xoo). Liquid chromatography–tandem mass spectrometry analysis of biologically active fractions from Xoo supernatants led to the identification of a 194–amino acid protein designated Ax21 (activator of XA21-mediated immunity). A sulfated, 17–amino acid synthetic peptide (axYS22) derived from the N-terminal region of Ax21 is sufficient for activity, whereas peptides lacking tyrosine sulfation are biologically inactive. Using coimmunoprecipitation, we found that XA21 is required for axYS22 binding and recognition. axYS22 is 100% conserved in all analyzed Xanthomonas species, confirming that Ax21 is a pathogen-associated molecular pattern and that XA21 is a pattern recognition receptor.