Microglia are known to play an important role in the CNS cytokine network, and their response after irradiation may be associated with the development of radiation-induced tissue damage. Radiation effects on this cytokine network have not yet been elucidated. We investigated the effect of gamma-irradiation on microglia stimulated with Zymosan A and lipopolysaccharide (LPS), which alone induce the expression of some cytokines and neurotoxic products by microglial cells. In the resting condition (ramified microglia), radiation had no effect on the mRNA level corresponding to cytokines such as IL1beta or IL-6, although TGF-beta1 mRNA was minimally enhanced by irradiation. However, in the activated microglia (amoeboid microglia) stimulated with Zymosan A, radiation-induced IL-6 mRNA expression was increased about two-fold in comparison with non-irradiation. IL-1beta was slightly induced by 2 Gy irradiation, but was not induced by higher doses. TGF-beta1 mRNA was not enhanced by radiation following Zymosan stimulation. In the LPS-stimulated condition, IL-6 mRNA was induced only by 2 Gy of irradiation, but no change in the expression of other genes was detected. These results suggested that radiation exerted different effects on cytokine gene transcription in microglia depending on their morphological state.