Toxicity of copper and folpet--two fungicides widely used on grape--was evaluated on Lemna minor L., a sensitive aquatic weed regularly used for (eco)toxicological studies. Toxicity assessments were based on inhibition of growth and chlorophyll content of L. minor cultures after 7 days. IC10, IC50, and IC90 were determined for both compounds alone and were respectively, 0.03, 0.16, and 0.95 mg liter-1 for copper and 1.20, 7.50, and>> 40 mg liter-1 for folpet. When both compounds were combined, the response of L. minor depended on the initial folpet concentration. Indeed, a slight synergy was observed for 5 mg liter-1 folpet, while at folpet concentrations of 20 to 35 mg liter-1, the two fungicides were antagonists. The antagonism was positively correlated with folpet concentration. Antagonism between Cu and folpet could not be explained by a reduced bioavailability of Cu since concentration of free copper in the mixture did not depend on the presence of folpet. One physiological defense response elicited by copper in plants is an increase in catalase activity. Copper and folpet stimulated catalase activity and changes in the activity of the enzyme could not account for the synergy but possibly for the antagonism. Nevertheless, catalase activity increase significantly after a 24-h exposure to 25 micrograms liter-1 of copper. The use of this property as a rapid and sensitive biomarker to monitor the toxicity of xenobiotics alone or in combination and of environmental water is discussed.