Investigates several microorganisms as potential indicators for the stability of pathogenic removal in water treatment plants (WTPs) in Korea. Details of a study to focusing on bacteria [E. coli, fecal coliform (FC), total coliform (TC) and heterotrophic plate counts (HPC)], protozoan [aerobic spore formers (ASF) and Clostridium perfringens (CP) spore], and viruses (F-specific RNA coliphage); Results suggesting that HPC and ASF appeared to be good indicators of the stability of pathogenic removal.