In recent years, federal statutes such as the Toxic Substances Control Act, the Resource Conservation and Recovery Act, and the Clean Air and Clean Water Acts have given the U.S. Environmental Protection Agency (EPA) the responsibility for regulating the release of toxic chemicals into the environment. In order to fulfill this function effectively, the EPA must first determine which of the thousands of chemicals currently in use or proposed for use are toxic.

Detecting a chemical's ability to cause immediate (or acute) toxic effects is a relatively straightforward task. Assessing the long-term (or chronic) toxic effects is much more difficult. Chronic effects such as cancer, birth defects, and genetic disease characteristically appear several years or decades after the initial chemical exposure has occurred, and long-term studies using live animals must be conducted in order to detect these latent effects. Such studies are expensive and time-consuming, and require the use of highly specialized facilities and personnel. A single test for a chemical's carcinogenicity (cancer-causing ability), for instance, may take as long as 3 years and cost $250,000 or more.

The number of compounds whose chronic toxicity has not been determined is overwhelming. Over 50,000 chemicals are currently in commercial production, and most of them have never been examined for chronic effects. The world laboratory capacity for long-term studies has been estimated at only 500 compounds per year, not enough to keep up with the 700 to 1,000 new chemicals that are introduced into commerce annually.

In response to this situation, short-term tests have been developed to serve as rapid and relatively inexpensive predictors of a chemical's potential to cause chronic effects. These tests employ bacteria, yeast, plants, insects, isolated mammalian cells and whole animals. Short-term tests can detect a chemical's genotoxicity, that is, its ability to alter a cell's genetic material (DNA). An increasing amount of evidence exists to indicate that latent diseases such as cancer, birth defects, and genetic disease may be initiated by alterations in the DNA.