The biological stability of treated drinking water is important to the U.S.E.P.A. and to water utilities because of concern for compliance with the coliform MCL. The level of assimilable organic carbon (AOC) in water is affected by water treatment. Treatment processes such as ozonation that increase the level of AOC, combined with lowered disinfectant residual to reduce disinfectant by-product formation, may result in increased bacterial growth in the distribution system. Information is needed on factors that impact AOC measurement and interpretation, and on strategies to control microbial growth in response to AOC.