Purpose: To analyze the proliferation rates and the radioscnsitivily of human bronchial epithelial cells (HBECs) with genetic alterations including human telomerase reverse transcriptase (hTERT), Cdk4 (cyclin dependent kinase 4), knockdown p53 and mutant K-ras overexpression. Materials and Methods: HBECs obtained from human bronchus spe¬cimens were placed into short-term culture and were serially transtected with relroviral constructs containing Cdk4 and hTERT, resulting in continuously growing immortalized normal human bronchial epithelial cell lines. These cell lines were used to produce knock-down p53, mutant K-ras and knockdown p53 plus K-ras expressing cell lines; the proliferation rates and the radiosensitivity of these cell lines were evaluated. The control cells and the Cdk4, hTERT, knockdown p53 and K-ras expressing cells were irradiated (1 Gy-10 Gy) and the fraction survival was determined 1 week later. Results: The proliferation rates of the cells expressing both knockdown p53 and K-ras were higher than those of the other cell lines and show a loss of contact inhibition in vitro. The HBECs infected with both knock¬down p53 and mutant K-ras were more radioresistant compared to cells infected with only mutant K-ras or knockdown p53. Conclusion: HBECs expressing knockdown p53 in addition to mutant K-ras have higher proliferation rates and additionally they lose the ability of growth arrest in response to DNA damage signals such as irradiation.