The ability of SV40 T antigen to block apoptosis was investigated in Rat1-A fibroblasts expressing an estrogen-dependent c-myc construct, mycER (Eilers et al., 1989). These RatmycER cells undergo apoptosis upon activation of c-myc by estradiol under conditions of serum deprivation. Under such conditions SV40-transfected derivatives of RatmycER undergo apoptosis as evidenced by rapid cell death, characteristic morphological changes and DNA fragmentation in a manner indistinguishable from the parental cell line, indicating that T antigen is not able to protect against myc-induced apoptosis. In as much as it had been reported that myc-mediated apoptosis involves wild-type p53 in other systems and T antigen is known to bind and inhibit p53 function, we examined these two polypeptides under different experimental conditions. In all cases, the great majority of the p53 in the SV40 transfectants was found to be in complexes with T antigen. Furthermore, the residual p53 in the uncomplexed state was not sufficient to transactivate an endogenous promoter, WAF1/p21. These data indicate that the failure of T antigen to block apoptosis cannot be attributed to defective function of T antigen and suggest that myc-mediated apoptosis may involve a p53-independent pathway in these cells.