The aim of the present work was to systematically study the effect of low concentrations of dissolved organic matter (DOM) on the bioconcentration of organic contaminants, in order to show whether the phenomenon of enhanced bioconcentration factors (BCFs), that has been reported in the literature, is generally found at low levels of DOM or if BCF enhancements are more likely due to a random scatter in the experimental data. The first part of the study tested the hypothesis that low levels of DOM affect the uptake kinetics of organic contaminants, leading to transient enhancements of BCFs, relative to DOM-free controls, which could have been reported as BCF enhancements in short-term studies. We found that the presence of low concentrations of two different types of DOM consistently decreased the bioconcentration of benzo[a]pyrene (BaP) in the water flea Daphnia magna at all exposure times (1-24 h), and that no transient BCF enhancements occurred. The second part of the study systematically investigated if low concentrations of DOM from a wide range of different aquatic systems can cause enhancements in the bioconcentration of organic contaminants. Water fleas were exposed to combinations of four different organic contaminants (BaP, tetrachlorobiphenyl, pentachlorophenol and naphthalene) with low concentrations of 12 different types of DOM that had been collected from various regions throughout Europe. In several of the DOM treatments, we found mean BCFs being higher than mean BCFs in the controls (especially for naphthalene). This shows that the experimental setup used in this study (and similarly in previous studies) can produce seeming BCF enhancements at low concentrations of DOM. However, statistical analyses showed that treatment means were not significantly different from control means. Thus, this systematic study suggests that the BCF enhancements that have been reported in the literature are more likely the result of random, experimental variations than the result of a systematic enhancement of bioconcentration. ?? 2001 Elsevier Science Ltd.