By crosslinking radioiodinated recombinant human IL1 alpha to mouse EL4 thymoma cells we have identified in addition to the known IL1-binding proteins of 80 kDa, a second IL1-binding protein of about 40 kDa. This second binding protein could be demonstrated most easily when crosslinking to higher protein complexes was inhibited. This finding suggests that the IL1 receptor, similar to the receptor for other cytokines such as interleukin 2, is composed of a heterodimer, of which both polypeptides contribute to ligand binding.