AbstractTransforming growth factor β (TGF‐β) is a cytokine with immunoregulatory properties that acts negatively on T lymphocyte proliferation. However, with the EL 4–6.1 variant of the murine thymoma EL 4 activated with phorbol ester and/or interleukin‐1 (IL‐1), we recently found that it up‐regulates interleukin‐2‐receptor (IL‐2R) expression. Since EL 4–6.1 cells share phenotypic and functional characteristics with the immature thymic subset lacking CD4 and CD8 accessory molecules (DN), we investigated the effect of TGF‐β1 on the IL‐2R 55kD α chain expression and proliferation of activated DN cells and especially in DN cells that do not express CD3. We observed that TGF‐β1 was able to increase both the percentage of CD3−DN cells expressing IL‐2Rα chains and the expression of IL‐2Rα chain in these cells. This stimulatory effect of TGF‐β1 was distal from early transduction events. In addition, TGF‐β1 was found to modulate CD3−DN cell proliferation. During differentiation in the thymus, CD3−DN cells transiently express the IL‐2Rα chain of the IL‐2R and these IL‐2R+CD3−DN cells are preprogrammed to down‐regulate the IL‐2Rα chain and up‐regulate the CD4 and CD8 accessory molecule. We thus also tested the effect of TGF‐β1 on IL‐2Rα chain expression in these in vitro differentiating CD3−DN cells. We found that TGF‐β1 neither significantly affected IL‐2R expression nor changed CD4 or CD8 expression. Hence, in CD3−DN cells, the effect of TGF‐β1 on IL‐2