In order to analyse the immortalizing and transforming potential of simian virus 40 (SV40), we compared the transformation efficiencies of SV40 in primary and in established BALB/c mouse fibroblasts. Five independently isolated clones of freshly immortalized normal fibroblasts (FTE cells) were established from precrisis BALB/c mouse embryo fibroblasts (pMEF cells) according to the protocol for establishing 3T3 cells (Todaro & Green, 1963). These cells expressed a wild-type p53 and were indistinguishable in all parameters analysed from original 3T3 cells kept in our laboratory. Using abortive infection to control gene dosage, followed by selection of transformed cells by cloning in soft agar, SV40 was able to transform primary cells with a much higher efficiency than 3T3 or FTE cells. Analysis of this unexpected result revealed that the different transformation efficiencies of SV40 in primary and established cells correlated with an altered cellular response to SV40 infection regarding metabolic stabilization of p53 complexed to large T during abortive infection. Whereas p53 in pMEF cells became stabilized upon abortive infection with SV40, p53 in 3T3 and FTE cells remained unstable. Our results strongly favour the hypothesis that metabolic stabilization and the ensuing higher levels of p53 in abortively infected cells enhance the transforming competence of large T.