Although a high dose thoracic radiotherapy is an effective strategy for some malignancies including lung cancers and malignant lymphomas, it often causes complications of radiation fibrosis. To study the mechanism initiating tissue fibrosis, we investigated irradiation-induced cytokine production from human lung fibroblastic cells and found that IL-6 production was stimulated by irradiation. IL-6 is an autocrine growth factor for human myeloma cells, and retinoic acid is reported to inhibit their growth. Thus, we evaluated the effect of all-trans retinoic acid (ATRA) on cell proliferation of lung fibroblasts along with the cytokine/receptor system. Irradiation-dependent stimulation of IL-6 production was correlated with increased NF[[kappa]B activity, and ATRA reduced this effect. Irradiation also increased the levels of mRNA for IL-6R and gp130, which were blocked by co-existing ATRA. Further, IL-6 stimulated cell proliferation in dose dependent manner, but was overcome by pharmacological concentration of ATRA. These effects of ATRA were inhibited by rottlerin, which suggests ATRA abolished irradiation-induced stimulation through PKC dependent pathway. Finally, we demonstrated that IL-6 transcripts in the lung were upregulated at two months after irradiation and the effect was inhibited by the intraperitoneal administration of ATRA. ATRA is expected to have an advantage for radiotherapy concerning its anti-tumor effects as reported previously, and prevention for radiotherapy-induced pulmonary injury.