Chloride intracellular channels (CLICs) are soluble, signal peptide-less proteins that are distantly related to Omega-type glutathione-S-transferases. Although some CLICs bypass the classical secretory pathway and autoinsert into cell membranes to form ion channels, their cellular roles remain unclear. Many CLICs are strongly associated with cytoskeletal proteins, but the role of these associations is not known. In this study, we incorporated purified, recombinant mammalian CLIC1, CLIC4 and (for the first time) CLIC5 into planar lipid bilayers, and tested the hypothesis that the channels are regulated by actin. CLIC5 formed multiconductance channels that were almost equally permeable to Na(+), K(+) and Cl(-), suggesting that the 'CLIC' nomenclature may need to be revised. CLIC1 and CLIC5, but not CLIC4, were strongly and reversibly inhibited (or inactivated) by 'cytosolic' F-actin in the absence of any other protein. This inhibition effect on channels could be reversed by using cytochalasin to disrupt the F-actin. We suggest that actin-regulated membrane CLICs could modify solute transport at key stages during cellular events such as apoptosis, cell and organelle division and fusion, cell-volume regulation, and cell movement.