To evaluate the effects of a p53-inducible gene WAF1/Cip1 on cell proliferation and apoptosis, a recombinant adenovirus vector (E1 minus) expressing WAF1/Cip1 cDNA (AdWAF1) was constructed and compared with a previously studied recombinant adenovirus vector expressing wild-type p53 (AdWTp53). Infection of normal and tumor cells of lung and mammary epithelial origin with AdWAF1 resulted in high levels of WAF1/Cip1 gene expression, which was comparable to that induced by AdWTp53. AdWAF1 and AdWTp53 inhibited growth of all cells studied; tumor cells devoid of endogenous p53 (H-358) or cells expressing endogenous mutant p53 (MDA-MB-231) were more sensitive to the inhibitory effect than tumor (MCF-7) or normal mammary epithelial cells expressing endogenous wild-type p53. Cell cycle analysis of AdWTp53-infected cells indicated a decline in the cell number in S phase and a significant increase in cell number in G2-M phase. AdWAF1 infection also led to a decline in the percentage of cells in S phase and a significant accumulation of cells in G1. AdWAF1 failed to induce apoptosis in any of the cells tested. In contrast, AdWTp53 induced apoptosis in H-358 and in MDA-MB-231 cells. These data suggest that AdWTp53-mediated WAF1/Cip1 induction and cytotoxicity are likely to be associated with WAF1/Cip1-mediated cell cycle arrest. However, because overexpression of WAF1/Cip1 protein failed to induce apoptosis, AdWTp53 effects on apoptosis apparently require cellular factors in addition to WAF1/Cip1 induction.