Due to organic N exiting in the pure bacterial cultures, when using free chlorine to inactivate microorganisms, freest chlorine transforms into organic N-chloramines which are no use in disinfection process. This reaction happens in E. coli, Pseudomonas aeruginosa inactivation experiments. Added 2 mg x L(-1) free chlorine to inactivate E. coli of 10(8) CFU x mL(-1), free chlorine exhausts in 5 min while chloramines and dichloramines become 0.92 mg x L(-1) and 0.4 mg x L(-1). E. coli is used as test microorganism in following inactivation experiments. E. coli suspension is purified by centrifugal and membrane filtration, but the purification can't help to separate the organic N from E. coli suspension, which indicating that the source of organic N may be from bacterium cells. Dilution is worked when E. coli concentrations are diluted under 10(5) CFU/mL. This result should be regarded in related researches in future.