Immunogold labelling was used to probe the responses of mesophyll cells in French bean (Phaseolus vulgaris L.) to anhrpAmutant ofXanthomonas campestrispv.vesicatoriaand a saprophytic strain ofX.c.The non-pathogenic strains both caused localized alterations to the plant cell wall and formation of large papillae in adjacent cells. Immunocytochemistry showed the co-localization, in the cell wall and paramural deposits, of anMr 42 000 proline-rich glycoprotein with chitin-binding activity (CBPRP) and the enzyme responsible for its immobilization, anMr 46 000 peroxidase. The CBPRP appeared to lose antigenicity after cross-linking, and, unlike the peroxidase, was not detected consistently in the extracellular matrix that encapsulated bacteria onto the plant cell wall. The peroxidase may have a dual function in both the generation and utilization of H2O2 for cross-linking of proteins and phenolics during the construction of papillae. A burst of H2O2 was detected 1–5 h after inoculation at reaction sites by histochemical staining with cerium chloride. Progressive expansion of papillae and cell-wall alterations was, however, not associated with the maintenance of high levels of H2O2. Co-localization of callose and anMr 65 000 polypeptide component of callose synthase was also demonstrated. Synthesis of callose appeared so rapid that the enzyme became embedded in the polysaccharide so that both were detected as integral to the developing papilla. Localized alterations to the cell wall and deposition of papillae were found to involve co-ordinated synthetic and oxidative activities at microsites within responding cells, without activation of the hypersensitive reaction.