Carbonic anhydrase (CA) is a metalloenzyme catalysing the reversible idratation of CO2 in Hþ and HCO3 . It is an ubiquitous enzyme in bacteria, plant and animal kingdoms, playing a fundamental role in a number of physiological processes. Previous studies demonstrated the sensitivity of CA activity to dichlorodiphenyl-dichloroethane (DDT) exposure in birds and to cadmium exposure in teleosts. The aim of the present work was to develop a new in vitro enzymatic bioassay for detecting toxic chemicals in environmental samples as a cost-effective tool in environmental monitoring. This bioassay uses the commercial available CA isozyme II from bovine erythrocytes whose sensitivity to the main classes of chemical pollutants of importance in water quality research was tested in this work. CA activity was determined by a modification of the electrometric method previously described by Wilbur and Anderson [K.M. Wilbur, G.N. Anderson. J. Biol. Chem., 176, 147 (1948).]: briefly, CA activity units were calculated from the rate of Hþ production in the reaction mixture (where CO2 was present as substrate) against a blank containing the specific CA inhibitor acetazolamide. [Hþ] variation was followed at 0C in the reaction mixture using a Mettler Delta 350 pH-meter. In our experimental set-up bovine CA activity was significantly inhibited by heavy metals (Cd, Cu and Hg), by the organochlorate compound arochlor and by the carbammate pesticides carbaryl in a dose-dependent manner. CA in vitro bioassay can represent a novel tool for rapid and low cost understanding of the toxicity of environmental samples, for assessing bioavailability of pollutants in environmental matrices and their additive or synergistic biological effects when present in mixtures.