Interleukin 1 (IL-1) is one of the major proinflammatory cytokines responsible for the regulation of immune and inflammatory response. In this paper we present up-to date information concerning the mechanisms of intracellular signal transduction induced by this cytokine. Interleukin 1 effects on target cells are mediated via membrane receptors. One of them (type I) is responsible for interleukin 1 intracellular signaling, whereas the other (type II) is a natural regulator of the cytokine activity.