Double negative (DN) T cells expanding in peripheral lymphoid tissues in mice bearing lymphoproliferation (Ipr) gene are generally unresponsive to mitogens, antigens, and anti-T cell receptor (TCR) or anti-CD3 monoclonal antibodies (mAb). In response to the stimulation with 0.125–5.0μM ionomycin, control T cells sustained an increase in intracellular free calcium ([Ca2+]i), while DN Ipr T cells showed a gradual fall following initial rapid increase in [Ca2+]i. Such gradual fall in [Ca2+]i was overcome by the addition of endoplasmic and sarcoplasmic reticulum Ca2+-ATPase inhibitor or high dose (10μM) of ionomycin. The requirement of high concentration of calcium ionophore for the sustained increase of [Ca2+]i in Ipr DN T cells is due to the dysfunction of Ca2+-ATPase pump.