Cytokines are proteins with multiple functions indispensable to body homeostasis. They are also good indicators of certain diseases which they are sometimes suspected to induce (e.g. septic shock, allograft rejection). Making cytokine assay methods available to scientists and clinicians has been a notable step forward in the understanding of numerous illnesses. However, these quantitative assays create more problems than they solve, and although reliable and easy tests exist we cannot recommend their use in routine clinical practice. In 1992, cytokine assays must be reserved for research studies where their results in patient populations are meaningful and where they can usefully be completed by other detection methods, such as amplification by polymerase chain reaction or hybridization in situ. Nevertheless, the constant improvement of assay kits, the ever increasing number of cytokines that can be quantified by simple laboratory tests and a better study of interactions between cytokines, their soluble receptors and their natural inhibitors will perhaps make it possible, in the near future, to analyse finely the immune response in not very invasive samplings. Moreover, there are hopes that modern molecular biology tools will soon be available in daily clinical practice. For instance, the development of automated methods for amplification by polymerase chain reaction will provide useful information on the activation of the immune system in various clinical situations. Such techniques, combined with measurements of local or circulating cytokine concentrations are undoubtedly promising tools. Finally, cytokine assays are essential to research workers who can study, within well-defined systems, the kinetics of production and the respective role played by these mediators.