Extracellular acidic pH-activated chloride channels (I-Cl,I-acid) have been found in a variety of mammalian cells. In the present study, the expression and regulation of I-Cl,I-acid were investigated in THP-1 cells. Patch clamp recordings demonstrated that an extracellular acidic solution induced an outward rectified current, which could be blocked by the Cl- channel blocker. The currents exhibited time-dependent facilitation and inactivation. The relative anion permeability of this current followed the sequence Cl- > Br- > I- > gluconate. NADPH oxidase inhibitors did not decrease pH 4.4-induced currents. However, reactive oxygen species (ROS) scavengers and mitochondrial inhibitors inhibited pH 4.4-induced currents. Fluorescence imaging of intracellular ROS and mitochondrial activity confirmed these findings. We conclude that I-Cl,I-acid occurs in human THP-1 cells and that I-Cl,I-acid may be regulated by intracellular ROS mainly originating from mitochondria. (C) 2013 Elsevier Inc. All rights reserved.