Treatment of the leaves of pepper (Capsicum annuum) cv. ECW10R with lipopolysaccharides (LPS) from both plant pathogenic and enteric bacteria alters several aspects of the plant response to subsequent inoculation with phytopathogenic xanthomonads. LPS pre-treatment prevents the hypersensitive reaction caused by strains of Xanthomonas campestris pv. vesicatoria carrying the avirulence gene avrBs1 (a gene-for-gene interaction) and by X. campestris pv. campestris (a non-host interaction). Associated with this effect are the earlier synthesis of feruloyl- and coumaroyl-tyramine, phenolic conjugates that are potentially antimicrobial, and alterations in the expression patterns of genes for some pathogenesis-related (PR) proteins. Similar effects on the timing of phenolic conjugate synthesis are also seen in the compatible interaction with X. campestris pv. vesicatoria, although the level of the response is lower. Recognition of LPS by plants may allow expression of resistance in the absence of catastrophic tissue damage. However phytopathogenic bacteria may have evolved mechanisms to suppress the effects of LPS (and of other non-specific bacterial elicitors) on plant cells.