Exposure of human cells to genotoxic agents induces various signaling pathways involved in the execution of stress- and DNA-damage responses. Inappropriate functioning of the DNA-damage response to ionizing radiation (IR) is associated with the human diseases ataxia-telangiectasia (A-T) and Nijmegen Breakage syndrome (NBS). Here, we show that IR efficiently induces Jun/ATF transcription factor activity in normal human diploid fibroblasts, but not in fibroblasts derived from A-T and NBS patients. IR was found to enhance the expression of c-Jun and, in particular, ATF3, but, in contrast to various other stress stimuli, did not induce the expression of c-Fos. Using specific inhibitors, we found that the ATM- and Nibrin1-dependent activation of ATF3 does neither require p53 nor reactive oxygen species, but is dependent on the p38 and JNK MAPkinases. Via these kinases, IR activates ATF-2, one of the transcription factors acting on the atf3 promoter. The activation of ATF-2 by IR resembles ATF-2 activation by certain growth factors, since IR mainly induced the second step of ATF-2 phosphorylation via the stress-inducible MAPkinases, phosphorylation of Thr69. As IR does not enhance ATF-2 phosphorylation in ATM and Nibrin1-deficient cells, both ATF-2 and ATF3 seem to play an important role in the protective response of human cells to IR.