Amendments to the Safe Drinking Water Act (PL-93-523) highlight the continuing problem of waterborne disease by mandating the EPA to promulgate: (1) Criteria by which filtration will be required for surface water supplies; and (2) disinfection requirements for all water supplies in the United States. There is interest on the part of the EPA in applying the Ct concept for determining the inactivation of Giardia lamblia cysts as a key criterion for determining exceptions to the filtration rule. If a utility, in addition to meeting other requirements, can demonstrate that through effective disinfection, manifested by a sufficient Ct value, it can reduce Giardia levels by 99.9%, then it will be exempted from surface water filtration. In the paper, a model for calculation of Ct values based on animal infectivity data is developed. The model, based on first-order kinetics, relates Ct values to chlorine concentration, pH and temperature. The model predictions are compared to laboratory data.