The most immature lymphoid-committed progenitors in both the bone marrow (common lymphoid progenitor) and thymus (proT1) maintain a latent granulocyte/macrophage (G/M) differentiation potential that can be initiated by signals emanating from exogenously expressed IL-2 receptors. In this study, we investigate at which developmental stage thymocytes lose this G/M differentiation potential. We demonstrate that the next maturational stage after proT1 cells (proT2), but not preT (TN3) cells, can convert cell fate from lymphoid to myeloid in response to ectopic IL-2 receptor signaling in human IL-2Rβ transgenic mice. It is significant that approximately 10% of clonogenic G/M colonies derived from proT cells of IL-2Rβ transgenic mice have DJ rearrangement specifically at the Dβ1 but not Dβ2 segment in the TCRβ locus. No TCR gene rearrangement is observed in G/M cells from nontransgenic mice, suggesting that the G/M cells we observe in this system were truly lymphoid-committed before stimulation with IL-2. In addition, Dβ1 and Dβ2 DJ rearrangement of the TCRβ gene may be differentially regulated and thus serve as markers for distinct proT cell maturational stages.