The p53-inducible cyclin-dependent kinase inhibitor, p21/WAF1/CIP1 (p21), plays a pivotal role in the G1 arrest or apoptosis of cells exposed to genotoxic stimuli. To determine whether p21 is a putative tumor-suppressor gene, p21 status was investigated in 4 human bladder-cancer cell lines of known p53 status. A p21-gene mutation, one base-pair insertion at codon 20 resulting in a chain-termination change at codon 35, was observed in one cell line, HT1376, suggesting structural or functional alteration of the p21 protein. When exposed to DNA-damaging agents, cisplatin or mitomycin C, apoptosis was induced in RT4 with the wild-type (wt) p53/wt p21, whereas T24 with the p53 non-sense mutation/wt p21 was resistant. Of the other 2 cell lines with the p53 mis-sense mutation, apoptosis was induced in SCaBER with the wt p21, but HT1376 with the p21 frame-shift mutation was fairly resistant. These findings suggest that not only p53 alteration, but also p21 alteration, is important to prevent apoptosis induced by DNA-damaging agents. When exposed to these agents, p53 and p21 expression was increased in RT4, and not induced in T24. p53 was not induced, but p21 expression was increased in SCaBER, whereas p53 expression was increased but p21 expression was absent in HT1376. Thus, p21 expression itself may have an important role in the induction of apoptosis by DNA-damaging agents.