The CDK inhibitor p21waf is a principal mediator of p53 function but can also be transactivated by many p53-independent stimuli leading to cell growth arrest or differentiation. In order to study the function of p21waf in a p53-deficient environment, we established an inducible expression of p21waf in the p53-null lung cancer cell line H1299, based on the muristerone-regulated system. Overexpression of p21waf led cells to growth arrest which after several days became irreversible and the arrested cells acquired a senescent phenotype as judged by cell shape, the senescence-associated beta-gal marker and inhibition of colony formation. The effect of p21waf overexpression, in the absence of p53, on the cytotoxicity caused by irradiation, doxorubicin and taxol was studied. Expression of p21waf provided protection against the cytotoxic effect of radiation and doxorubicin but not of taxol. These results are relevant to treatment of cancer when p53 is inactive.