Anticholinesterases constitute a major portion of modern synthetic insecticides and the assessment of cholinesterase (ChE) inhibition is widely used as a specific biomarker for evaluating the exposure of non-target organisms to these pollutants. To evaluate the possible exposure of the North Adriatic sea coastal environment to residues of agriculture practices, we ascertained whether the oyster, Ostrea edulis and the clam, Tapes philippinarum, highly valuable resources commercially harvested in the area, could be selected as sentinel species. We characterized the ChE biochemical properties in their gills, and for comparison the analysis was carried out also in gills of the mussel, Mytilus galloprovincialis, extensively used as a biological indicator of water quality. In the oysters and mussels, the ChE activity was a function of increasing concentrations of substrate in the range 0.01-0.5 mM. K(m) values were 93+/-15 and 77+/-8 microM for O. edulis and M. galloprovincialis, respectively. No detectable ChE activity was found in gills of T. philippinarum, at any tested condition. Significant inhibition of ChE activity by eserine, carbaryl, and ethyl-paraoxon was observed in gills of O. edulis and M. galloprovincialis. Although field validation is needed, the present study suggests that O. edulis may be employed as a biological indicator for assessing pesticide contamination, whilst T. philippinarum does not seem useful for this purpose.