Highly boron-doped diamond (BDD) electrodes were modified covalently with tyrosinase for the determination of estrogenic phenol derivatives, BDD was anodically polarized for the introduction of hydroxyl groups onto its surface, then treated with (3-aminopropyl)triethoxysilane and finally coated with a tyrosinase film cross-linked with glutaraldehyde. The modified electrodes responded amperometrically to phenol derivatives including estrogenic derivatives, bisphenol-A and 17beta-estradiol. at -0.3 V versus Ag/AgCl. Interference from direct reduction of oxygen at the electrode surface was almost negligible because the overpotential for the oxygen reduction at BDD was greater than those for most conventional electrode materials. The electrodes were applied to a flow injection system. and the lower detection limit for bisphenol-A was 10 (6) M. The stability and proper sampling interval were also studied.