Bcl-2, Bax and p53 gene products have been linked to programmed cell death pathways. p21WAF1 has been shown to mediate p53-induced cell cycle arrest and to inhibit cyclin-dependent kinase activity. We have analysed the expression of these genes and apoptosis induced by 12-O-tetradecanoyl-phorbol-13-acetate (TPA) in several human breast cancer cell line. We found up-regulation of p21WAF1 and Bax expressions, however, the expressions of p53 and Bcl-2 genes remained unchanged in TPA-treated cells. Furthermore, DNA ladder formation and PARP cleavage were observed after treatment for 24 h, indicating apoptotic cell death. Flow cytometry with 7-amino actinomycin D staining showed that the number of apoptotic cells increased with longer treatment of TPA. From these results, we conclude that TPA is not only a tumor promoter, but also induces apoptosis in breast cancer cells. TPA-induced apoptosis appears to be mediated through a p53-independent pathway, and the up-regulation of p21WAF1 and Bax may be the molecular mechanisms by which TPA induces apoptosis.