The purpose of this study was to adapt the ethoxyresorufin O-deethylase (EROD) assay to the microplate technique. In addition to showing the practical advantages related to the intrinsic properties of such systems, we wished to demonstrate their potential value for biological monitoring of contamination in natural environments. A simpliffed procedure was developed for the preparation and fluorimetric assay of EROD activity using only basic laboratory equipment. The use of plate readers allows measurements to be done rapidly and provides numerous advantages in comparison with traditional methods. Within the context of our study; this test could be substituted to advantage for the enzymelinked immunosorbent assay (ELISA).