In neutrophils, Ca2+ influx across the plasma membrane occurs in response to a variety of agonists. It plays an important role in the regulation of neutrophil function, in particular exocytosis and release of inflammatory mediators. Using the combination of patch clamp and microfluorimetry techniques, we have found the following characteristics of neutrophil Ca2+ influx: (a) neutrophils do not possess voltage-dependent Ca2+ channels; (b) Ca2+ influx is activated by depletion of intracellular Ca2+ stores, either by Ins(1,4,5)P3, or by Ca(2+)-ATPase inhibitors; (c) Ca2+ influx is not activated by [Ca2+]i elevations and not associated with the activation of non-selective cation channels; (d) Ca2+ influx is associated with the activation of a small, voltage-independent current through a highly Ca(2+)-selective conductance. The current was only seen upon depletion of intracellular Ca2+ stores and showed a pharmacological profile similar to the fluorimetrically detected Ca2+ influx.