In severe infections two factors play a part: the infectious agent and the response of the host. The response of the host involves production of a large number of endogenous mediators including a number of cytokines that are currently the focus of many studies: tumor necrosis factor (TNF alpha), interleukins (IL-1 and IL-6), and interferon gamma (IGN gamma). These cytokines are part of the body's normal defense mechanisms but can have toxic effects when produced in excessive amounts. Although levels of these cytokines are often high in the blood of patients with sepsis, persistence of these elevations is the main indicator of severe infection. Experimentally, injections of TNF alpha and IL-1 reproduce the manifestations of severe sepsis. Mice that are genetically unable to produce TNF alpha are resistant to the injection of endotoxin. Severe sepsis can be prevented by pretreatment of animals with anticytokine agents (polyclonal and monoclonal antibodies and anti-receptor agents ...). Many issues remain unresolved: for instance, the experimental results obtained with an intravenous bolus of endotoxin or bacteria have not been confirmed in some animal models of subacute infection. These models may more closely resemble human infections. The interrelations between these cytokines are extremely complex. Synergistic effects do occur, but the effects of combinations of cytokines can be different from those of each cytokine given alone... It follows that therapeutic use in humans of anti-cytokine molecules is still an approach of uncertain outcome that will perhaps be clarified by ongoing multicenter clinical trials.