We previously demonstrated the existence of a third component, p64, of IL-2 receptor (IL-2R), tentatively named the gamma chain of IL-2R. Our recent studies provided evidence suggesting that the gamma chain endows the beta chain of IL-2R with IL-2 binding ability. The gamma chain was detected in lymphoid transfectants of IL-2R beta cDNA, which showed the intermediate-affinity IL-2R, but not in nonlymphoid transfectants of IL-2R beta cDNA, which showed no IL-2 binding activity. The comparative study between two subclones of lymphoid MOLT4 transfectant of IL-2R beta cDNA demonstrated that the amount of the gamma chain coprecipitated with IL-2R beta was proportional to numbers of the IL-2 binding sites. These results suggest the possibility that the gamma chain associates with IL-2R beta and has an important role in formation of the intermediate-affinity IL-2R complex. On the other hand, we have also demonstrated the association of IL-2R beta with a certain tyrosine kinase, of which activation by IL-2 could be indispensable process at the initial pathway of signal transduction.