Telomerase is associated with cell proliferation capacity, protection and stabilization of chromosomes. TA (telomerase activity) can be detected in highly replicative cells, e.g. stem and cancer cells. Most available mESC (mouse embryonic stem cell) research is done with a few cell lines. The purpose of this study has been to evaluate the TA in different passages of newly isolated mESC. TRAP (Telomeric Repeat Amplification Protocol)-ELISA method was used in a semi-quantitative evaluation of TA. Three mESC lineages were investigated (CT2, CT3 and CT4) at three different passages (P13, P15 and P19). In contrast with previous studies, these mESC lines did not show the same TA throughout their passages, having initially low TA values, followed by a subsequent rise and stabilization.