In vitro effects of binary mixtures of various DOM (mainly HS) with various HOCs on AhR-mediated responses were studied by use of H4IIE-luc cells. 6 out of 12 DOM activated the AhR even at environmentally relevant concentrations (17 mg L(-1)). In simultaneous exposures of H4IIE-luc cells to DOM and each of the model compounds, 2,3,7,8-TCDD, PCB126, PCB169, benzo[a]pyrene, benzo[a]anthracene, dibenz[a,h]anthracene, fluoranthene, a mixture of POPs, a mixture of PAHs, and a mixture of all HOCs, either significant additive or facilitative effects were observed when compared to activities of single HOCs. No significant decrease of effects due to sorption of HOCs to DOM was observed, even in subsequent experiments when HOCs + DOM mixtures were preincubated for six days before exposure to H4IIE-luc. Thus, DOM does not seem to protect organisms against AhR-mediated toxic effects of HOCs (as predicted due to sorption of HOCs on DOM), but it can enhance their potency for AhR-mediated effects in some situations.