AbstractMouse fibroblasts do not ordinarily express components for the interleukin‐2 receptor (IL‐2Rα, β, and γ). An analysis of these cells by reverse transcription followed by polymerase chain reaction, however, indicates the presence of transcripts specific for the IL‐2R β and γ genes. Transfection of the cDNA for the α chain of the human IL‐2R into LTK− mouse fibroblast cell line (L3 cells) leads, in long‐term cultures, to the formation of transcripts of endogenous mouse IL‐2, IL‐2Rα and β genes, as detected by Northern blotting. Based upon the results of the binding of 125I‐labeled IL‐2 to the transfected cells, three IL‐2‐binding proteins of 55 kDa, 65 kDa and 75 kDa were expressed by the transfected cells. The 65‐kDa and 75 kDa proteins bound IL‐2 in the presence of monoclonal antibodies for the IL‐2Rα chain. These polypeptides assembled to form high‐affinity IL‐2R, as shown by Scatchard binding analyses. The receptors were functionally active, since the expression of H‐2k major histocompatibility complex antigens on the surface membranes of L3 cells was enhanced by exposing the cells to IL‐2. Activation of the IL‐2 gene was also observed in long‐term cultures of Lαβ cells, another LTK− transfectant expressing the human IL‐2Rα chain. This type of gene activation was not observed in LTK− fibroblasts transfected with cDNA for human IL‐2 or IL‐2Rβ genes. In L3 and Lαβ cells, transcription of the endogenous IL‐2 gene was suppressed by cyclosporin A and enhanced by cycloheximide. These data may have implications for gene therapy of cancer cells.