Nine samples of coagulated and filtered river water were addnL purified by adsorption on activated C, artificially polluted with natural humic substances (org. C content 2-6 mg/L) and with NH2Cl (NH3 content 0.1-1.0 mg/L), disinfected with Cl 2 under tab. conditions and filtered through Amberlite XAD sorbent beds to remove Cl-contg. org. compds. Presence of mono, di and trichloroacetic acids and other chloroorg. compds. in the Cl-disinfected water was evidenced. The compds. were recovered by Me,,CO leaching and tested for mutagenic activity on Salmonella typhimurium TA100 strain (Ames assay) both with and without metabolic activation with a microsomal fraction. Only the water samples contg. 2 mg/L of org. C showed no mutagenic effect irrespective on the NH3 concn. and metabolic activation.