Exposure of mammalian cells to ultraviolet (UV) light and other DNA‐damaging agents triggers the UV response which is characterized by induction of a large number of genes including c‐fos, c‐jun, and the genes for DNA repair enzymes and cell‐cycle regulatory proteins such as p21^WAF1 and p53. Upon DNA damage, the p53 tumor suppressor protein transmits signals to restrict cell‐cycle progression, thereby allowing time for DNA repair to occur. Cells also respond to genotoxic stress by activation of the jun N‐terminal kinase (JNK)/stress‐activated protein kinase pathway. In this report we investigated the effects of modulation of the level of wild‐type and mutant p53 protein on basal and UV‐inducible JNK activity. We used the A1‐5 rat fibroblast cell line, which contains a p53 gene coding for a temperature‐sensitive p53 protein, which allows us to regulate the relative level of wild‐type and mutant p53 protein produced in a cell. We measured the relative levels of JNK activity in sham‐irradiated and UV‐irradiated cells by using the immune complex kinase assay and then computed the fold induction of JNK after UV exposure. We demonstrated that cells expressing p53 protein in the wild‐type conformation (when grown at 32°C) exhibited a very low level of JNK activity that was induced 14‐ to 16‐fold by UVC irradiation. When cells were grown at 37°C or 39°C to express predominantly mutant p53 protein, basal JNK activity was significantly higher than at 32°C. UVC irradiation of cells expressing mutant p53 protein resulted in JNK activation, although the overall fold‐induction was only two‐fold because JNK1 activity was already high in the sham‐treated controls. UVB irradiation also induced JNK1 activity, although we again observed a relatively high level of basal JNK activity in sham‐irradiated cells expressing mutant p53 protein compared with cells expressing wild‐type p53. Control experiments confirmed that JNK1 basal activity was not affected by temperature alone. Western blot analysis of cell extracts indicated that expression of p21^WAF protein was significantly higher in cells expressing wild‐type p53 protein and was associated with low basal levels of JNK1 activity. In contrast, cells expressing mutant p53 protein and very low levels of p21^WAF1 protein were found to have a higher level of basal JNK1 activity. We also observed a reduced ability to induce JNK1 after UV irradiation of several other cell lines with p53‐mutant or p53‐null genotypes. Our results provide evidence for a novel connection between p53 status and the basal level of JNK1, a critical enzyme in the stress‐activated protein kinase family. In addition, these studies suggest that the presence of mutant p53 protein in a cell not only affects basal activity of JNK1 but also affects the ability of a cell to respond to UV‐induced stress by transmitting signals via induction or activation of the JNK1 cascade. Mol. Carcinog. 25:262–272, 1999. © 1999 Wiley‐Liss, Inc.