Lipopolysaccharides (LIPS), ubiquitous cell surface components of Gram-negative bacteria, are directly implicated in plant/pathogen interactions. However, their perception by the plant, the subsequent signal transduction in both compatible and incompatible interactions, as well as the defence reactions induced in compatible interactions are as yet poorly understood. We focused on biochemical and physiological reactions induced in cell suspensions of three Solanaceae species (tobacco, tomato, and potato) by purified lipopolysaccharides from Pectobacterium atrosepticum (Pa), a pathogen of potato, and Pseudomonas corrugata (Psc), a pathogen of tomato. LPSPa, and LPSPsc caused a significant acidification of potato, tomato, and tobacco extracellular media, whereas laminarin (a linear beta-1,3 oligosaccharide elicitor) induced an alkalinisation in tobacco and tomato, but not in potato cell suspensions. None of the two LIPS induced the formation of active oxygen species in any of the hosts, while laminarin induced H2O2 production in cells of tobacco but not of tomato and potato. In tomato cells, LIPSPa and LPSPsc induced a strong but transitory stimulation of lipoxygenase activity, whereas laminarin induced a stable or slightly increasing LOX activity over the first 24 h of contact. In tobacco, LOX activity was not triggered by either LPS, but significantly increased following treatment with laminarin. In potato, neither LPS nor laminarin induced LOX activity, in contrast with concentrated culture filtrate of Phytophthora infestans (CCF). These results demonstrate that LPS, as well as laminarin, are perceived in different ways by Solanaceae species, and possibly cultivars. They also suggest that defence responses modulated by LPS depend on plant genotypes rather than on the type of interaction