Since H4IIE-luc results represent an overall AhR activity, extract purification as well as fractionation prior to exposure do largely affect the final result. In contrast, as long as there are no interferences, the cleanup does not affect the chemical measurements (Van Wouwe et al., 2004). Cleanup of environmental extracts such as sediments is necessary for this bioassay, since the presence of cytotoxic or antagonizing compounds can result in a false negative response or a false low induction (Behnisch et al., 2001a).