b-Amyloid (Ab) peptides accumulate in the brain in Alzheimer's Disease (AD) and are implicated in its pathogenesis. Microglia, the immune cells of the CNS, are activated by Ab and in response, produce reactive oxygen species (ROS) through activation of the plasmalemmal NADPH oxidase. Generation of ROS by microglia is thought to contribute to the cell death seen in AD. We have previously shown that Ab activates a chloride current mediated by the protein CLIC1 (Novarino et al., 2004). The present work highlights attempts to clarify the mechanisms through which this current might be related to microglial-mediated ROS production. Using primary microglial cultures and BV2 cells, we have measured the effects of Ab on microglial membrane potential and calcium signalling, and particularly rates of ROS generation using dihydroethidium fluorescence. While application of Ab peptides to microglia increases the rate of dihydroethidium oxidation, such a response is not seen when the reverse peptides are used or when the NADPH oxidase is inhibited. The increased ROS production effected by Ab was also dependent on the presence of CLIC1. Suppression of CLIC1 protein expression using siRNA, inhibition of the CLIC1 chloride current using IAA-94 or using an antibody to the channel protein, and replacement of extracellular Cl- with impermeant anions, all significantly reduced the ROS response to Ab.