The 1986 Amendments to the Safe Drinking Water Act (SDWA) require 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 (MCLs) or treatment requirements for control of Giardia lamblia viruses, Legionella, hetertrophic plate count, bacteria, and turbidity. Giardia lamblia is one of the most resistant organisms to disinfection by chlorine found in water and therefore the Surface Water Treatment Rule specifies 'C.t' (the product of Concentration of disinfectant in mg/L and disinfectant contact time in minutes) for 99.9 percent inactivation of Giardia cysts. Many factors influence Giardia lamblia reaction kinetics including temperature, pH, chlorine concentration and inactivation level. The paper develops a model to describe these interactions and to predict C.t values based on specific model inputs. A strategy is proposed that uses the model to provide conservative C.t values for regulatory purposes.