Previously, we reported that SV40 T/t-common polypeptide, which contains the NH(2)-terminal common domain of SV40 large T and small t antigens, can repress HER2/neu (also known as erbB-2) expression and consequently suppress the tumorigenic potential of the HER2/neu-overexpressing ovarian carcinoma cells. Here we report that T/t-common could specifically induce apoptosis in HER2/neu-overexpressing human cancer cell lines but not in nontransformed cell lines and HER2/neu low-expressing human cancer cell lines. The ability of T/t-common to induce apoptosis in HER2/neu-overexpressing cancer cells was derived from its ability to inhibit HER2/neu because reexpression of a large amount of HER2/neu could block apoptosis induced by T/t-common. T/t-common expression in HER2/neu-overexpressing SK-OV-3 cancer cells led to down-regulation of Bcl-2 and Bcl-X(L), and overexpression of Bcl-2 could inhibit the ability of T/t-common to induce apoptosis in these cells. Therefore, the apoptosis-inducing activity of T/t-common is related to its ability to inhibit Bcl-2 expression in HER2/neu-overexpressing cancer cells. Consistent with the apoptosis-inducing activity of T/t-common, we found that T/t-common could specifically inhibit the soft-agarose colony-forming ability of the HER2/neu-overexpressing human cancer cell lines but not that of the HER2/neu low-expressing human cancer cell lines. Finally, we showed that T/t-common could specifically sensitize HER2/neu-overexpressing human cancer cell lines, but not HER2/neu low-expressing human cancer cell lines, to chemotherapeutic agent etoposide. Together, these data suggest that T/t-common alone or in combination with chemotherapy may provide a new approach for treatment of cancers that overexpress HER2/neu.