The experimental investigation of effluent from municipal wastewater treatment plants and nightsoil treatment plants was conducted from the viewpoint of safe effluent water quality to evaluate the strategies and regulations for wastewater reuse. A bacterial assay named umu-test, which can detect error-prone repair dependent on DNA damage by some chemicals, was applied to samples from treatment plants. Concentrates of secondary effluent of municipal sewage treatment plant revealed strong genotoxicity, and its activity could be observed every day of the week. Also, the samples from secondary effluent separated by ultrafiltration in a nightsoil treatment plant showed positive genotoxicity. Newly developed strains which are Salmonella typhimurium in umu-test are highly sensitive to aromatic amines and can detect the genotoxic activity induced by these compounds. By using this method, the genotoxic potency of above samples was examined, The toxicity on aromatic amines could be detected in the matter contained in raw nightsoil. This shows that human feces involve some genotoxic substances and the genotoxicity could not be reduced through the biological treatment with nitrification and denitrification, nor removed by the ultrafilter separation process. A coagulation-sedimentation process could partially remove the toxic substances from the solution and activated carbon adsorption could almost remove the substances. The liquid chromatography column was applied to fraction the concentrated substances to elute the toxic substances from samples. Some genotoxic fractions related to aromatic amines would be observed more clearly by this method. In order to reuse the effluent from the treatment plant of the very close side to human body, the reduction of those genotoxicities is expected from the viewpoint of human health. From the results of ozone application to the effluent, it was found that the genotoxicity could be clearly reduced to a negative level at the ozone consumption ratio of 1 mgO(3 consumed)/mgC.