Ca2+ is an essential second messenger for T cell activation, but the exact mechanisms of its action are poorly understood. The cytosolic Ca2+ concentration is significantly increased upon the stimulation of T cells with either mitogen, cross-linking antibodies, or their cognate ligands. In this study, expression of calreticulin, a major Ca(2+)-binding (storage), KDEL protein of the endoplasmic reticulum was examined in resting and concanavalin A (ConA)-stimulated mouse and human T-lymphocytes. Both resting, mouse and human lymphocytes contain very low levels of calreticulin mRNA and protein. Mouse splenocytes stimulated with ConA exhibited an induction in calreticulin mRNA which peaked by Day 4. A 5-fold increase in the immunoreactive calreticulin protein band was also observed in the cells during this period of stimulation. Similarly when human lymphocytes were cultured with ConA a significant increase in the levels of the calreticulin mRNA and protein was observed. The peak of calreticulin mRNA was observed at Day 1 rather than Day 4 as seen for the mouse. These results clearly demonstrate the presence of calreticulin, a Ca(2+)-binding protein originally characterized in muscle tissue, in activated T-lymphocytes. Furthermore, we show that expression of calreticulin correlates with T-lymphocyte activation. Our results suggest that calreticulin may be involved in the signaling pathway for the induction of Ca(2+)-dependent processes and may represent one regulatory mechanism operating in activation of T-lymphocytes.