We have tested the dividing cells in the mouse thymus for expression of interleukin 2 (IL-2) receptors (IL-2-R) using the rat monoclonal antibody 7D4. A discrete subpopulation of the lymphoblasts clearly expressed IL-2-R at levels comparable to those on mitogen-activated peripheral T cells. This subpopulation, however, represented a small minority of the proliferating cells. IL-2-R-bearing cells were depleted from the PNA+ (peanut agglutinin) lymphoblast population, which contains the direct precursors of most of the cells in the thymus. The majority of receptor-bearing cells were found in the PNA- lymphoblast population, where they constituted only approximately 12% of the cells. Thus, virtually all the PNA+ and most of the PNA- blast cells were in cycle without detectable IL-2-R expression. This indicates that they were not dividing in response to IL-2, and implies that they were not dividing in response to antigen, but rather to novel thymus-specific mitogenic stimuli. On the other hand, the proliferating cells that do express IL-2-R were enriched 4-5-fold in the rapidly growing neonatal thymus, suggesting that they may also play a key role in T cell development.