Ion transport from one aqueous (W1) to another (W2) across bilayer lipid membranes (BLM) containing gramicidin A (GA) was investigated by recording current fluctuations, when various alkali metal chlorides and potassium salts were used as supporting electrolytes. The magnitude of the single-channel current at a given membrane potential depended on not only the cationic species, but also on the anionic species, and then it decreased with an increase in the diameter of the anion when the diameter of the anion was larger than the pore size of the GA channel. The baseline of the recording current, however, increased with an increase in the diameter of the anion, and its height depended on the concentration of GA in the BLM. The results indicate that GA serves as not only a channel-forming compound, but also as a carrier compound in the BLM.