To understand the molecular mechanisms that underlie radiation pneumonitis, we examined whether knockout of the TNF or the IL-6 gene could give mice an inherent resistance to radiation in the acute phase of alveolar damage after thoracic irradiation. The temporal expression of inflammation (CD44) and apoptosis (Bak) markers in lung after thoracic irradiation was measured to determine the degree of alveolar damage. At 4 weeks post-irradiation (10 Gy), small inflammatory foci were observed in all mice, but there were no obvious histological differences between control (C57BL/6JSlc), TNF-alpha knockout (TNF KO), and IL-6 knockout (IL-6 KO) mice. However, immunohistochemical analysis of CD44 and Bak expression over a time course of 2 weeks highlighted significant differences between the three groups. C57BL/6JSlc and TNF KO mice had increased numbers of both CD44-positive and Bak-positive cells after irradiation, while the IL-6 KO mice showed stable levels of CD44 and Bak. In conclusion, the radioresistant status of IL-6 KO mice in the acute phase of alveolar damage after irradiation suggested an important role for IL-6 in radiation pneumonitis.