A mouse monoclonal antibody (mAb) to the rat interleukin-2 receptor beta (IL-2R beta) chain was generated using IL-2R beta cDNA-transfected mouse L929 cells for immunization and differential screening. This antibody, called L316, detects a cell surface protein with an apparent molecular mass of about 80 kDa. In peripheral lymphoid organs of young adult rats, IL-2R beta expression is restricted to T and natural killer (NK) cells, and less than 10% of IL-2R beta+ cells co-express the IL-2R alpha chain. IL-2R beta was detected on all NKRP-1hi (NK-) and NKRP-1lo cells (T-lineage cells of unknown function), most peripheral gama delta T cells and on 30-40 % of CD8 and 10 % of CD4 alpha beta T cells. In the adult rat thymus, mAb L316 detects a small subset (about 1%) of predominantly IL-2R alpha- cells which express cell surface markers characteristic of mature T lymphocytes and contain a high proportion of CD4-8- and CD4-8+ alpha beta T cell receptor (TCR)+ thymocytes. TCR-V usage suggests that major histocompatibility complex (MHC) class I plays a more important role than MHC class II in the selection of these cells. On immature CD4+8+ rat thymocytes, IL-2R beta cell surface expression is readily induced by TCR stimulation in vitro, supporting the idea that in vivo, the IL-2R beta+ phenotype is the the result of TCR engagement during thymic selection.