This report deals with a test water distribution system used to evaluate the stability and effectiveness of free chlorine, combined chlorine and chlorine dioxide when challenged with sewage. Total coliform bacteria and seeded f2 bacterial virus were used to determine rates of inactivation. This study concludes that the primary barrier against post-treatment contamination is the presence of residual disinfectant in the distribution system. Free chlorine was found to be the most effective residual disinfectant. Free chlorine served as a reliable marker for contamination. The absence of free residual chlorine, in a system in which it was normally maintained, was evidence that chlorine-demand substances had entered the system. This marker capability did not hold for combined chlorine residuals. Free chlorine and chlorine dioxide were the least stable disinfectants, with difficulties being encountered in maintaining residuals under low-flow conditions. At levels of 0.2 to 1.0 mg per liter, for all these disinfectants, no surviving enteric virus were concentrated from the systems contaminated with detectable levels from raw sewage.