Interleukin-1 (IL-1) is a polypeptide hormone produced by a wide variety of cells in response to immunologic challenge. It appears to play a vital role not only in the generation of an appropriate immune response but also in the pathogenesis of several disease states. The mechanism(s) by which it exerts a number of biological effects on target cells are poorly understood. At least one membrane receptor for IL-1 has been cloned and purified, although it is clear that other receptor proteins exist that have yet to be characterized completely. The current state of knowledge of the biochemistry of IL-1 receptor proteins and their possible function is discussed, with particular emphasis on transmembrane signalling in T lymphocytes.