c-jun-N-terminal kinases (JNKs) play an important role in defense against external stresses including ionizing radiation (IR). We have previously shown that sensitivity to IR is influenced by p53 status in human thyroid cells. In this study, we investigated the effect of p53 status on IR-induced JNK activation in human thyroid cells. Our results showed high basal JNK activity in the p53-null thyroid cancer cell line, FRO. In contrast, primary cultured thyroid cells (PT), which harbor wild-type p53, had low basal JNK activity. IR increased JNK activity in PT, however, no such increase was noted in FRO cells. Introduction of the wild-type p53 into FRO cells reduced JNK activity to a low basal level and rendered it responsive to IR. There was no difference in IR-induced ceramide production between PT and FRO cells. Our results provide clear evidence that p53 status influences, directly or indirectly, radiation-induced JNK activation in human thyroid cells, suggesting that a feedback or interaction pathway between p53 and JNK regulates radiation-induced cell fate.