Purpose: To address the functional role of radiation-induced transforming growth factor-β (TGF-β) signaling in a normal epithelial background, we selected a spontaneously immortalized lung epithelial cell line derived from the normal lung tissue of a dominant-negative mutant of the TGF-β RII (ΔRII) transgenic mouse that conditionally expressed ΔRII under the control of the metallothionein promoter (MT-1), and assessed this cell line’s response to radiation.Methods and Materials: A spontaneously immortalized lung epithelial cell culture (SILECC) was established and all analyses were performed within 50 passages. Colony-forming and terminal transferase dUPT nick end labeling (TUNEL) assays were used to assess clonogenic inhibition and apoptosis, respectively. Western-blot analysis was performed to assess the kinetics of p21, bax, and RII proteins. Transforming growth factor-β-responsive promoter activity was measured using dual-luciferase reporter assay.Results: Exposure to ZnSO 4 inhibited TGF-β signaling induced either by recombinant TGF-β1 or ionizing radiation. The SILECC, treated with either ZnSO 4 or neutralizing antibody against TGF-β, showed a significant increase in radio-resistance compared to untreated cells. Furthermore, the expression of ΔRII inhibited the radiation-induced up-regulation of the TGF-β effector gene p21 waf1/cip1 .Conclusions: Our findings imply that inhibition of radiation-induced TGF-β signaling via abrogation of the RII function enhances the radio-resistance of normal lung epithelial cells, and this can be directly attributed to the loss of TGF-β signaling function.