Pre-treatment of leaves of pepper (Capsicum annuum) with lipopolysaccharide (LPS) preparations from enteric bacteria and Xanthomonas campestris could prevent the hypersensitive response caused by an avirulent X. campestris strain. By use of a range of deep-rough mutants, the minimal structure in Salmonella LPS responsible for the elicitation of this effect was determined to be lipid A attached to a disaccharide of 2-keto-3-deoxyoctulosonate; lipid A alone and the free core oligosaccharide from a Salmonella Ra mutant were not effective. For Xanthomonas, the core oligosaccharide alone had activity although lipid A was not effective. The results suggest that pepper cells can recognize different structures within bacterial LPS to trigger alterations in plant response to avirulent pathogens.