Mutagens dispersed in ecosystems are usually in low concentration and episodic in occurrence. The possibility of detecting such dispersed mutagens by utilizing indigenous bioconcentrator organisms coupled with a microbial mutagen assay offer a useful screening protocol. There are numerous examples of plant and animal species which concentrate toxic substances from the environment. Body extracts of these bioconcentrators can be suitably fractioned and tested for mutagens with various microbial mutagen assays. The fractions may be tested with a broad range of microbial assays covering numerous genetic end points as well as both with and without mammalian microsomal activation. This kind of environmental screening has an advantage over physicochemical techniques, in that sampling techniques are simpler and a wider chemical spectrum can be screened. There are problems inherent with testing a complex biological extract, however. If a reversion assay is used, the metabolite necessary for growth may be present. Toxins may be introduced, either concentrated from the environment in the same way as the mutagen, or produced by the concentrator itself. Finally, the concentrator may also produce an endogenous mutagen which will give spuriously active extracts. Methods for minimizing some of these difficulties are discussed.