Activated T lymphoblasts respond more effectively to mitogenic stimuli than resting T cells, partly through differences in Ca2+ signaling, which in turn depend on K+ channel activity. Both Kv1.3 and Ca2+-activated K+ (SK4) channels are up-regulated in T lymphoblasts. Since Ca2+- and calmodulin (CaM)-dependent signal-ing are key pathways in T-cell activation, we investigated their involvement in regulating the Kv1.3 current. Kv1.3 in lymphoblasts was significantly inhibited by elevating internal Ca2+ to the micromolar level. It was also reduced in a Ca2+-dependent manner by inhibiting CaM with W–7 or calmidazolium. Part of the CaM-dependence is likely through CaM kinase since the current was also inhibited by the antagonist, KN–62, but not by the inactive analogue, KN–04. Kinase inhibition, unlike CaM inhibition, was only effective at physiological temperatures, a difference that implies involvement of more than one mechanism. We demonstrated a biochemical association of Kv1.3 protein in lymphoblasts with the multifunctional type II CaM kinase, but not with calmodulin. Thus, Kv1.3 forms a multi-protein complex with CaM kinase II (which binds to Ca2+/CaM) and previously identified proteins (e.g., PSD–95, src tyrosine kinase) that position the channel to respond to signaling pathways that are crucial for T-cell activation and proliferation.