Information on the required chlorine dose or Ct value (concentration of free chlorine multiplied by contact time) is limited to only a few enteroviruses. In this study the Ct values of some of the reported more chlorine resistant enteroviruses were determined. The Efficiency Hom Model was used to predict the times for 2, 3. and 4 -log inactivation of echovirus 1 and 12, coxsackievirus B5 and poliovirus type 1 at pH 7.5 and 9 at 5 degrees C. Coxsackievirus B5 was the most resistant to chlorine with a Ct requirement of 11.5 mg x min/L at pH 7.5 at 5 degrees C compared to polio with a Ct of 5.3 mg x min/L under the same conditions. All the viruses were more resistant than polio 1 at pH 9.0.