In vivo bioassays are frequently used to assess the ecotoxicological risks of contaminated sediments. For quality assurance purposes, these bioassays are accompanied by reference toxicity tests. For the bioassay with Corophium volutator, this reference toxicity test is an acute water-phase test with cadmium. Approximately 80 water-phase tests were conducted in the period 1991-1998. Corophium volutator shows a significant seasonal variation in response to cadmium, with a high LC50 in the winter period and a low LC50 in the summer period. Same variation can be found if Corophium is held in the laboratory instead of freshly collected in the field and if synthetic water is used instead of natural filtered seawater. The observed seasonal variation is not caused by the fact that the organisms are collected in the field or by the variation in seawater used for the water-phase tests.