The spectrum of p53 mutations differs among human cancer types. We have hypothesized that the p53 mutational spectrum observed in particular tumor types reflects the functional ability of different p53 mutants to modulate wild-type (WT) p53-dependent gene transcription. Missense p53 mutants representing several mutational hotspot codons were cotransfected with WT p53 and analysed for their effects on p53-dependent transactivation of a reporter construct containing a specific p53 binding sequence (PG13-CAT) in human tumor cell lines lacking endogenous p53. Our results show that the ability of p53 mutants to inhibit WT p53-mediated transactivation is cell type dependent. In cell lines derived from a lung adenocarcinoma and a mesothelioma, the transactivation function of WT p53 was strongly inhibited by all p53 mutants examined. However, in cell lines derived from a prostate carcinoma and an osteosarcoma, the mutants examined generally had only minimal dominant negative effects. In cell lines derived from a hepatocellular carcinoma and an ovarian carcinoma, two mutants (248trp and 273his) enhanced WT p53-mediated transactivation of the reporter construct. Additional mutants retained the ability to inhibit WT p53-mediated transactivation in these cell lines. In addition, in a series of four breast tumor cell lines, the p53 mutants examined had similar effects on WT p53 transactivation ability including enhanced transactivation activity in the 273his cotransfectants. The p53 mutants were incapable of transactivating the PG13-CAT reporter in the absence of WT p53 expression. Therefore, the dominant negative effects of p53 mutants on WT p53 function may vary depending on the particular cell type. In addition, mutants with stronger inhibitory capabilities may confer a selective advantage during the tumorigenic process.