In response to DNA damage, the transcriptional activity of p53 rises. This has been thought to be due to an increase in the level of p53 protein. By comparing the p53 protein level and its ability to transactivate target genes Waf1/Cip1 and mdm2 in both T22 and NIH3T3 cells irradiated with u.v., a discordance between the p53 protein level and its transcriptional activity was observed. When the cells were irradiated with 10 J/m2 of u.v., there was a substantial increase in expression of Waf1/ Cip1 and mdm2. However, little increase in Waf1/Cip1 and mdm2 expression was observed in T22 and NIH3T3 cells 8 or 9 h after exposure to 50 J/m2 of u.v., although the p53 protein level accumulated to its highest level under these conditions. Interestingly, a significant increase in Waf1/Cip1 expression was seen 24 h after irradiation in NIH3T3 cells, indicating that the inhibition of p53 transcriptional activity is reversible. Discordance between the transcriptional activity of p53 and its protein level was further studied using a cell line expressing the p53 reporter plasmid RGC delta fosLacZ. Using double immunofluorescence staining, the coexpression of p53 and beta-galactosidase from the reporter plasmid in the same cells was investigated. The observed lack of correlation between the elevated p53 and beta-galactosidase and expression in u.v. irradiated cells strongly indicates that the ability of p53 to transactivate its target genes is not simply correlated to its protein level. The results indicate that the transcriptional activity of p53 may be negatively regulated.