Dextran sulfate sodium (DSS) solution was orally administered to mice to experimentally induce colitis, and the effects of X-ray irradiation of the spleen on healing or prolongation of inflammation was investigated. Peripheral, splenic and thymic T lymphocyte subsets were also analyzed to assess the underlying morbid conditions. Colitis with inflammatory changes consisting mainly of hemorrhagic erosions at the distal large intestine was induced by the administration of a 2 % DSS solution for 14 consecutive days. The DSS-induced colitis was inhibited by pre-irradiation of the spleen with 20 Gy prior to the development of DSSinduced colitis. Healing of inflammed lesion was accelerated by irradiation with 20 Gy during the induction of colitis with DSS. The proportion of peripheral Thy-1^+ and L3T4^+ cells was higher while the proportion of splenic L3T4^+ cells was significantly lower compared with the control group during the preparation of DSS-induced colitis. At 2 and 7 days after pre-irradiation, peripheral and splenic Thy-1^+ and L3T4^+ cells increased and thymic Lyt-2^- . L3T4^+ cells increased, whereas thymic Lyt-2^+ . L3T4^+ cell decreased. Our results suggest that the effect of splenic irradiation on DSS-induced colitis is due to a stimulation of T cell function, and activation of the immune system
