The use of bioassays in ecological risk assessments often raises questions about the causative factors, and insight into the possibility that confounding factors, such as pH or increased ammonia concentrations, might be responsible for the observed toxicity is needed. It was decided to develop a practical approach for the Dutch situation, in which a first screening is carried out based on provisional criteria. In collecting the required data, dozens of experiments were performed, while the scientific literature was searched for additional information. It is concluded that the provisional criteria specified are at present useful tools in interpreting results of bioassays. Depending on the outcome and the aim of the research, it might be necessary to further reduce uncertainties in the interpretation. This might require some additional experiments, using alternative controls or test procedures or altering the composition of the original sample.