Characteristics of estrogenic effect formed by chlorination of drinking water were investigated focusing on chlorination by-products. The MVLN assay was carried out to detect estrogenic effect. It was suggested that 17 β-estradiol would be a possible chemical that contributes to the estrogenic effect detected in the Lake Biwa water. It was speculated, however, that the effect of 17 β-estradiol is greatly suppressed by coexisting with organic matter in the Lake Biwa water. The main factor affecting the increase of estrogenic effect by chlorination would be the effect of chlorination by-products. The estrogenic effect of the Lake Biwa water was reduced by coagulation and granular activated carbon treatment. The estrogenic effect disappeared almost after activated carbon treatment. The reduction of the estrogenic effect by granular activated carbon treatment was consistent with the removal of TOC. In the meantime, the estrogenic effect of water after coagulation and activated carbon treatment increased by ch lorination. The important result is to find that there is a problem equal to the production of trihalomethanes even in the estrogenic effect of drinking water. The result shows that the estrogenic effect is formed as a result of the reaction of chlorine with organic matter remained after water treatment.
