Despite the wealth of information on the regulation of wild-type p53 function by phosphorylation, nothing is known about the biological effect of phosphorylation on mutant p53. Here we show that p53H175 is phosphorylated like wild-type p53 in cells of the same background. Ser(392) nonphosphorylatable p53 mutants p53H175A392 and p53W248A392 more potently transformed rat embryo fibroblasts in cooperation with the ras oncogene than p53H175S392 and p53W248S392. p53H175A392 also had an enhanced ability to confer cellular resistance to the cytotoxic effect of cisplatin and UV radiation. This correlated with p53H175A392 being a more potent dominant negative mutant than p53H175 in inhibiting the apoptotic functions of wild-type p53. Moreover, p53H175E392, which mimics the phosphorylated form of p53H175, was less able to confer cellular resistance to DNA-damaging agents. p53H175 and p53W248 are phosphorylated like wild-type p53 in cells of the same background. Ser(392) nonphosphorylated p53 was present in human breast tumors expressing mutant p53 including p53H175. Together, these results demonstrated a novel function of Ser(392) phosphorylation in regulating the oncogenic function of mutant p53.