Lipopolysaccharides (LPS) are cell surface components of Gram-negative bacteria and, as microbe-/pathogen-associated molecular patterns, have diverse roles in plant-microbe interactions, e.g. LPS are able to promote plant disease tolerance through activation of induced or acquired resistance. However, little is known about the mechanisms of signal perception and transduction in response to elicitation by these bio-active lipoglycans. The present study focused on the involvement of LPS isolated from the outer cell wall of the Gram-negative bacterium Burkholderia cepacia (strain ASP B 2D) in the molecular mechanisms and components involved in signal perception and transduction and defense-associated responses in suspension-cultured tobacco (Nicotiana tabacum L.) cells. The purified LPS(B.cep.) was found to trigger a rapid influx of Ca2+ into the cytoplasm of aequorin-transformed tobacco cells. An oxidative burst, concomitant with the production of reactive oxygen and nitrogen species was measured by chemiluminescence and fluorescence. These early perception responses were accompanied by K+/H+ exchange and alkalinization of the extracellular medium. Through the use of various inhibitors of the oxidative burst reaction, as well as scavengers of produced radicals, the biochemical basis of the cellular response to LPS(B.cep.) elicitation was dissected, elucidated and compared to that induced by a yeast elicitor. These results suggest that LPS(B.cep.) interacts with tobacco cells in a manner different from the response elicited by yeast elicitor.