Lipopolysaccharides (LPS, endotoxin) are known to be responsible for the initiation of endotoxic shock, therefore they can be targets for new preventive and therapeutic strategies. This review describes the biological and physicochemical properties of endotoxin ? major component of the outer membrane of Gram-negative bacteria. The paper focuses on response of the host to endotoxin, recent knowledge about the target cells and receptors for LPS, mechanisms of innate immune response and cell signal transduction due to the LPS recognition. It also summaries results of studies on LPS structure influence on its biological activity.