The influx of Ca2+ across the T lymphocyte membrane is an essential triggering signal for activation and proliferation by an antigen. The aim of this study was to determine if Ca2+ influx through estradiol receptor (ER) operated channels of Ca2+ entry induced activation of lymphoid cells. Mouse thymocytes were incubated with 17 beta -estradiol (E) and in the presence or absence of the mitogen, phytohemagglutinin (PHA), Despite evidence of an enhanced binding of E to ER on thymocyte membranes, and an E dose-related influx of Ca2+, there was a consistent down regulation of IL,-2 receptor expression IP < 0,001). Incubation of thymocytes with PHA enhanced IL,-2 receptor expression although the down regulatory effect of E was still evident. The results suggest that the Ca2+ channel activated by E may have a down regulatory effect on the IL-2 receptor in thymus cells leading to the dampening of cell activation process.