The 1986 amendments to the Safe Drinking Water Act require the U. S. Environmental Protection Agency (EPA) to promulgate primary drinking water regulations (1) specifying criteria under which filtration would be required, (2) requiring disinfection as a treatment technique for all public water systems, and (3) establishing maximum contaminant levels or treatment requirements for control of Giardia lamblia, viruses Legionella spp., heterotrophic plate count bacteria, and turbidity. EPA has promulgated treatment technique requirements to fulfill the Safe Drinking Water Act requirement for system using surface waters and groundwaters under the direct influence of surface water (Federal Register, 1989). Additional regulations specifying disinfection requirements for systems using groundwater sources not under the direct influence of surface water will be proposed and promulgated at a later date. A model has been developed that relates pH, temperature, chlorine concentration, and inactivation level to Giardia inactivation by free chlorine. It was found that C times T (the product of disinfectant concentration (milligram per liter) and disinfectant contact (minutes) values increased with level of inactivation, disinfectant concentration and pH and decreased with temperature.