The quality and biological effects of the waste waters released by Baikalsk Pulp and Paper Mill were monitored chemically and biologically. In 1:5 dilution, the treated waste water induced monooxygenase activity in the liver of grayling and bullhead in standardised laboratory conditions. The Ames test was used to monitor mutagenicity of waste waters of the mill and tissue extracts of fish, sponge, mollusc and plankton collected near the mill and of seals collected from the middle part of the lake. Mutagens were found in waste water produced during pulp chlorination before treatment and in a few samples of fully treated waste water during 10 years (1982–1993). Bottom sediments near the mill showed some mutagenicity. Mutagens were not found in tissues of most aquatic animals studied with the exception of zooplankton, roach and seals collected in Lake Baikal close to the discharge outlet of the mill.