A water sample from Tama River was concentrated into one f ourhundredth volume by rotary evaporator. A half of the concentrate was fractionated into five pools by gel chromatography on a column of Sephadex G-25. Another half of the concentrate was chlorinated with sodium hypochloride and fractionated into five pools by the same procedure. These pools were lyophilized. Neither of these pools showed mu tagenic activity at a maximal dose of 3 mg/plate when these pools were assayed by the Ames

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... nella test in the persence or absence of S-9 mixture. Humic acids which are considered as a prominent component in some of natural water were prepared from a water sample from Kushiro River. This preparation did not show mutagenic activity while a commercial product of humic acids showed low but definite activity.