The enzyme telomerase plays an important role in sustaining the capacity of T lymphocytes for homeostatic replication. Recent data have suggested that gonadal steroids might modulate telomerase expression or activity within these cells. We used quantitative assay techniques for both telomerase mRNA expression and telomerase enzymatic activity to systematically examine the effects of physiologic concentrations of estradiol on human peripheral blood mononuclear cells under basal conditions and under conditions that normally enhance telomerase activity in T lymphocytes. Cells from women tended to exhibit higher responsiveness of telomerase activity to induction by T cell receptor engagement. However, we found no evidence of a direct effect of physiologic concentrations of estradiol on human telomerase reverse transcriptase (hTERT) mRNA expression, hTERT protein expression, or telomerase enzymatic activity in cultured PBMCs. While estrogen might exert developmental effects on T cells to alter telomerase responsiveness to T cell receptor engagement, mature peripheral T cells do not respond to estradiol with changes in expression or function of telomerase.