Ca2+ activated Cl- channels (CaCCs) participate in a variety of physiological processes such as sensory transduction, electrical excitability, control of cell volume and transepithelial transport. Bestrophins have been recently proposed to form a new family of Cl- channels. We examined if these channels play a role for the Ca2+ activated Cl- secretion in epithelial cells. We demonstrate that the ATP induced short circuit current (Isc) in mouse trachea was inhibited by the blocker of CaCCs, niflumic acid (NFA). In whole cell recordings we detected Ca2+ activated Cl- currents (CaCIs) in various epithelial cell lines expressing endogenous bestrophin 1 (Best1). These currents were absent in cells lacking Best1 expression. RT-PCR, Western blot, immunostaining and whole cell patch clamp studies revealed the presence of both endogenous Best1 expression and CaCIs in renal, airway and colonic epithelia. Short interfering RNA (siRNA) for Best1 suppressed ATP induced whole cell currents in HT29 cells by 70%. Western blot analysis confirmed reduced expression of Best1 in siRNA transfected cells. CaCIs were also reduced in mouse M1 collecting duct cells, when a mouse Best1 antibody was present in the patch pipette. These results strongly suggest a role of bestrophin 1 for the CaCIs in epithelia. Best 1 may itself form the Cl- channel or may be part of a complex together with other membrane proteins. Supported by DFG KU756/9-1 and Else-Kröner-Fresenius Stiftung