A bioassay involves use of a biological organism to test for chemical toxicity. A bioassay is used to test the effects of compounds being considered for use in drugs or skin care products. Before a chemical compound receives health authorities approval as an ingredient in products for human use, it must be thoroughly tested on laboratory animals. For environmental testing, bioassays provide an integrated picture of overall toxicity of an effluent or a sample of water, sediment, or soil from a contaminated site. Fathead minnows, various aquatic invertebrates, earthworms, protozoans, and seeds all are used for bioassays of aquatic samples (see Keddy et al., 1995, for an extensive review). The idea behind these bioassays is that the test organism will react in a predictable way to various types of environmental contaminants. Several studies have compared the sensitivies of various types of seeds to common pollutants (for example, Wang and Williams, 1988; Wang, 1987; Wang, 1986).
Cornell University - http://ei.cornell.edu/toxicology/bioassays/Uses.html