The biological effects of acute toxicity, chronic toxicity, carcinogenicity, etc. of chlorination by-products detected in drinking water in Japan are discussed. The biological effects of representative chlorination by-products such as trihalomethane, haloacetic acid, haloaldehyde, haloacetonitrile, chlorophenol, chloropicrin, etc. as well as the evaluation of mutagenicity in drinking water purification process, for which Ames Salmonella/microsome assay is used for safety evaluation of drinking water, are discussed. The extent of the contribution of mutagenicity of chlorination disinfection by-products to the mutagenicity of drinking water is investigated. It must be admitted that biological evaluation of the safety of water quality is impossible currently by using only the known chemical substances contained in drinking water. The effects of chlorination and ozone treatment which are often applied to drinking water treatment are different each other. 58 refs., 1 fig., 4 tabs.