During the Bremerhaven Workshop cholinesterase measurements in dab Limanda limanda muscle were evaluated as a monitoring tool to assess the effect of pollutants along a 360 km transect in the North Sea, and around a drilling site. The basic properties of cholinesterases, together with their natural variability related to sex and size, were investigated. The results show the presence of at least 2 different enzymes, acetylcholinesterase and butyrylcholinesterase, with high activities in brain, muscle and liver. No Variation was observed in relation to sex or size. The activity of both acetylcholinesterase and butyrylcholinesterase was depressed in nearshore stations along the transect and no variation was observed around the drilling site. The K(m) of acetylcholinesterase from muscle of dab varied along the transect. The results lead us to interpret enzyme variation as the result of effects of neurotoxic compounds coming from the Elbe and Weser rivers into the German Bight, and validate cholinesterase activity as a tool for biological monitoring at sea.