Many tumors overexpress mutant forms of p53. A growing number of studies suggest that the nature of a p53 mutation in a cell can impact upon cellular properties, clinical responses to therapy and prognosis of a tumor. To explore the cellular basis of these observations, experiments were designed to compare the properties of cells with and without p53 mutations within the same cell population. To that end, various tumor-derived human p53 mutants were introduced into p53-null H1299 lung adenocarcinoma cells. Clonogenic survival assays revealed that cells overexpressing the p53His175 mutant, but not the p53His273 mutant, recover preferentially from etoposide treatment. Moreover, p53His175 as well as p53His179 reduced substantially the rate of etoposide-induced apoptosis, whereas p53His273 and p53Trp248 had a much milder protective effect. In contrast, p53His175 and p53His273 exerted very similar effects on the cellular response to cisplatin; both conferred increased resistance to low concentrations of the drug (2.5 microg/ml), but did not protect at all against high concentrations (10 microg/ml). Hence particular p53 mutants may confer upon tumor cells a selective survival advantage during chemotherapy. These findings define a new type of mutant p53 selective gain of function, which may compromise the efficacy of cancer chemotherapy.