The magnitude of the threat to the human genetic material posed by environmental agents has not as yet been fully determined. Never theless, the potential hazards of many chemicals have been identi fied by studies on lower organisms. However, too little is known regarding the comparability or lack of it between the metabolic pathways available in such organisms and those in man. Although at present there is great public concern for what is considered by some as the excessive use of laboratory animals in toxicological testing, it seems clear that the usage of mammalian systems may be deemed necessary. It has been proposed that cell culture systems might suffice to meet this need, however, such approaches cannot match the complexity of physiological occurances that are present in the intact animal. For studies of genetic effects, some non-invasive human test systems are presently available. These do not, however, meet the re quirements for extensive laboratory studies. In order to assess the risks to humans of environmental factors such laboratory investiga tions are essential. Therefore, for the forseeable future reliance on experiments using laboratory animals will be necessary. This Volume, which contains the proceedings of a workshop which was held at the National Institute of Environmental Health Sciences, March 29-31, 1982, explores the existing methodologies and their utility for risk estimations. It covers the most well developed human systems, as well as the most widely used animal tests.