An anion channel of sarcoplasmic reticulum vesicle has been incorporated into planar lipid bilayers by means of a fusion method and its basic properties were investigated. Analysis of fusion processes suggested that one SR vesicle contained approximately one anion channel. The conductance of this channel has several substates and shows a flickering behavior. The occupation probability of each substate was voltage dependent, which induced an inward rectification of macroscopic currents. Further, the anion channel was found to have the following properties. (1) The single-channel conductance is about 200 pS at 100 mM Cl-. (2) The channel does not select among monovalent anions but SO2-4 hardly permeates through the channel. (3) SO2-4 added to the cis side (the side to which SR vesicles were added) inhibits Cl- current competitively in a voltage-dependent manner. (4) An analysis of this voltage dependence suggests that the binding site of SO2-4 is located at about 36% of the way across the channel from the cis entrance.