The degradation of 17 beta-estradiol (E2) in water by TiO2 photocatalysis was investigated; concurrently the estrogenic activity of the treated water was evaluated during the photocatalytic reactions by an estrogen screening assay. As a result, 10(-6) M of E2 was totally mineralized to CO2 in 1.0 g L-1 TiO2 suspension under UV irradiation for 3 h. 10 epsilon-17 beta-Dihydroxy-1,4-estradien-3-one and testosterone-like species were elucidated as intermediate products by GC/MS analysis. The mechanisms of E2 degradation by TiO2 photocatalysis were discussed not only experimentally but also theoretically by calculating the frontier electron densities of the E2 molecule. On the basis of the results obtained, it was concluded that the phenol moiety of the E2 molecule, one of the essential functional groups to interact with the estrogen receptor, should be the starting point of the photocatalytic oxidation of E2. This means that the estrogenic activity should be almost lost concurrently with the initiation of the photocatalytic degradation. Actually, the estrogenic activities of the intermediate products were negligible. TiO2 photocatalysis could be applied to water treatment to effectively remove natural and synthetic estrogens without producing biologically active intermediary products.