Abstract. radiation-induced pulmonary injury is a severe complication affecting the quality of life of patients. although the pathophysiology of the process is not fully understood, we hypothesized that it is potentially related to macrophages and their secretion of matrix metalloproteinase-9 (MMP-9). Macrophages are a type of inflammatory cell that synthesize hundreds of bioactive substances and enzymes. MMP-9 is closely involved in the maintenance of the basilar membrane, and leads to increased extracellular matrix deposition within the lung, which is a characteristic feature of radiation-induced lung fibrosis. We examined the role of ionizing radiation in modulating the production of MMP-9 in a macrophage cell line. RAW264.7 cells were irradiated with various doses of γ-rays, and then MMP-9 and tissue inhibitor of metalloproteinase-1 (TiMP-1) levels were determined at several time points. rT-Pcr revealed a marked increase in the levels of MMP-9 mRNA, which peaked at 24 h post-irradiation and had begun to decline by 48 h. By contrast, TIMP-1 mRNA experienced only a slight increase at 24 h post-irradiation, reaching significance at 48 h post-irradiation. Western blot analysis demonstrated an increased expression of MMP-9 protein in the irradiated cells, while TiMP-1 protein levels were not notably changed. dexamethasone inhibited the increased expression of MMP-9 protein induced by ionizing radiation. These results indicate that MMP-9 expression by RAW264.7 cells, and an imbalance between MMP-9 and TiMP-1, may be involved in radiation-induced lung injury.