Septic shock remains an acute condition with a bad prognosis and a high mortality rate. This could be related to our incomplete understanding of the pathophysiological mechanisms involved, especially in the immunological field. Recently, several studies have stressed the key role of cytokines. Amongst these, the tumour necrosis factor (TNF) seems to be the most important. This peptide is a hormone secreted by monocytes and macrophages under the effect of various stimuli such as lipopolysaccharides or endotoxin. Giving TNF mimicks the clinical and biological patterns of septic shock. Moreover, high concentrations of TNF have been found in patients suffering from septic shock. Pretreatment with monoclonal antibodies against TNF prevents the occurrence of septic shock after endotoxin administration. TNF acts directly via ubiquitous specific receptors; this probably explains its diffuse activity. The therapeutic implications of these recent advances are not clear. It is not known, for the moment, whether TNF secretion is beneficial or deleterious for the patient.