Senescent phenotype can be attained by diverse agents, thus suggesting that there might be molecular differences between the senescence achieved in vivo and the senescence-like state attained in vitro under culture conditions. In this study we compare the senescent phenotype reached by cells derived from young animals when cultured in vitro with the one associated with the in vivo aging process. Several in vitro senescence parameters, including MTT reduction, proliferation rate, DNA synthesis, SA-beta-gal staining, and both in vivo and in vitro Bcl-2 content, were determined. Alterations in DNA electrophoretic mobility were evaluated to test differences in bulk chromatin structure. Our results indicate that although it is possible to achieve a senescent phenotype with cells derived from young animals aged in culture, this phenotype differs from the one observed in older animals, due to lack of in vivo damage inducers to which cells are being exposed during natural aging.