The mutagenicity of bleached pulp mill effluents was compared to the mutagenicity of unbleached waste waters. Mutagenicity was assessed with the Ames test, using metabolic activation systems isolated from the liver of rats and fish. Liver extracts from fish caught in polluted areas, and from fish experimentally exposed to waste water, were also investigated. Pulp mill effluents taken after chlorination showed mutagenic activity. The activity diminished during the waste water treatment. Tissue extracts from fish exposed to various concentrations of treated bleached and unbleached pulp mill effluents showed only slight mutagenic activity in a few samples. In the case of bleached pulp mill effluents monooxygenase activities were elevated in those samples where slight mutagenicity was observed. In the case of unbleached effluents no such correlation was found.