Using anti-murine interleukin-2 receptor beta chain (IL-2R beta) monoclonal antibody (mAb), we have examined the expression of IL-2R beta on murine thymocyte subpopulations. We found that it was constitutively expressed on 1%-4% of thymocytes in an almost mutually exclusive fashion with IL-2R alpha. The expression of IL-2R beta is developmentally regulated. While it is expressed mainly on T cell receptor gamma delta+ (TcR gamma delta+) cells during fetal age, the major subpopulation expressing IL-2R beta in adult mouse shifts to CD4-CD8-TcR alpha beta+ thymocytes. A considerable portion of CD4-CD8- TcR alpha beta+ cells in other organs, including spleen, bone marrow and liver, was also found to express IL-2R beta. In fetal thymus organ culture, the above thymocyte subset was induced to expand in response to exogeneous IL-2, and the expansion was inhibited by addition of anti-IL-2R beta mAb, suggesting that IL-2R beta is functional in this subpopulation. However, in vivo blockade of the IL-2/IL-2R pathway with the mAb did not exert any effects on the appearance of CD4-CD8- TcR alpha beta+ cells both in the thymus and the periphery. This indicates that the development of CD4-CD8- TcR alpha beta+ cells is not solely controlled by IL-2 but also by other complex elements.