Changes in time-course and dose-effect relationship of the DNA-binding activity of NF-,κ,B in thymocytes and splenocytes after whole-body irradiation (WBI) were observed using the method of electrophoresis mobility shift assay (EMSA). Results showed that there existed two types of dimers binding specifically to the ,κ,B site in the nuclear extracts of the thymus and spleen. The radiation-induced changes in time-course and dose-effect relationship of these two types of dimers were significantly different. The DNA-binding activity of p50/p65 in thymocytes only slightly increased after WBI with 2Gy, but that of p50/p50 started to increase at 1h after irradiation and reached its peak at 4h, and keeped at high levels until 12h. At 12h after irradiation, the changes in the binding activity of p50/p65 in thymocytes were not obvious for all doses, while the binding activity of p50/p50 showed a increase with the dose increase at the dose above 1Gy, suggesting that maybe the later complex with transcription repression activity could be chiefly induced by medium or high doses. Lower doses of radiation caused slightly the expression of p50/p65 and higher doses resulted in its depression in the spleen, and there was hardly any influence on the activity of the p50/p50 dimer in this organ. The results suggest that there may exist differences in compositions of NF-,κ,B complexes in different immune organs and differences in their reaction to ionizing radiation.