Changes in cytosolic Ca2+ are an important element in the activation of the immune response, and store-operated calcium channels (SOCs) are a major calcium influx pathway in immune cells. We have studied the role of intermediate-conductance calcium-activated K+ channels (K(Ca)3.1; SK4) in regulating Ca2+ influx through SOCs in human monocyte-derived macrophages. After depletion of intracellular Ca2+ stores by application of thapsigargin or UTP typical inwardly rectifying Ca2+ -relase activated Ca2+ -currents (ICRAC ) could be recorded with the patch-clamp technique. Using RT-PCR, we found orai1, stim1 and K(Ca)3.1 to be expressed in macrophages. With low intracellular Ca2+ buffering power, activation of store-operated Ca2+ influx produced an outward current with the typical properties of IK(Ca)3.1. In the current-clamp mode, store- operated Ca2+ -influx produced a strong hyperpolarisation. Both effects could be inhibited by charybdotoxin (100 nM) or by the ICRAC blocker 2-APB (50 mM). Fluorometric measurement of intracellular free Ca2+ with fluo-3 showed that at negative membrane potentials (5 mM external K+ ) the Ca2+ influx through SOCs was larger than at 0 mV (145 mM external K+ ). We propose that in human macrophages Ca2+ influx through SOCs gives rise to a net outward current through K(Ca)3.1 channels. The resulting hyperpolarisation enhances Ca2+ -influx and contributes to the fine-tuning of the immune response.