The spontaneous occurrence of coliforms in chlorinated drinking waters has resulted in concern over their potential source and mechanism(s) of introduction into water delivery systems. Previous observations related to protozoal resistance to chlorine coupled with the ingestion of bacteria by this group of organisms prompted an in-depth study of bacterium-protozoan interaction in the presence of chlorination. Bacteria used were either from previous 'spontaneous coliform outbreaks' or potential human pathogens. The protozoans utilized were laboratory adapted and wild strains of protozoans found naturally in water reservoirs. These organisms were established in axenic cocultures and the effect of various concentrations of chlorination and exposure were determined. It was found that, alone, the bacteria were killed by from 0.25-1 mg/L of chlorine; however, when ingested by protozoans, these same bacteria survived in significant concentration levels of chlorination 4 to 10 times the above dose. It further suggests a need for increased awareness in developing improved methods of treating reservoir waters.