We have previously shown that some types of synthetic zeolites can enhance protein synthesis in a coupled cell-free transcription/translation system derived from Escherichia coli. As a result of efforts to elucidate the mode of action of zeolite on protein synthesis, we found that enhancement mainly resulted from the ion exchange reaction induced by the addition of the zeolite. Even in the absence of zeolite, the yield of protein synthesis in the cell-free system was markedly increased by replacing the potassium ion in the reaction mixture with sodium ion.