At 5 degrees C in phthalate buffer at pH 4.5, poliovirus 1 was destroyed 60 to 70% more rapidly by free chlorine (FC) in the presence of 1,262 mg/L (approximately 0.0169 M) KCl than in the absence of KCl. At 5 degrees C, more than 99.9% of FC exists as HOCl at pH 4.5. Since buffers themselves may have a marked potentiating effect on the virus-inactivating capability of FC, and because a maximum ion potentiating effect may occur, the actual degree of potentiation of HOCl by KCl may be much greater. Because the level of chlorination for drinking water is in large part determined by the rate at which the chlorines inactivate viruses in laboratory studies, the potentiation of FC by ions normally present in drinking waters may permit reducing chlorination levels and with them the levels of carcinogenic and otherwise toxic organic halides produced by chlorination.