In the rapidly developing field of environmental mutagenesis, the use of stand- ardized protocols has permitted the evaluation of large numbers of chemicals for mutagenic and potential carcinogenic activity. In this sixth volume of Chemical Mutagens, the chapters by Perry, Williams, Rosenkranz and Leifer, Kada et al., Pienta, Tazima, Epler, and Rinkus and Legator provide a systematic listing of the chemicals that have been tested, bringing together both unpublished and previously published data. These chapters will serve as an important milestone by providing a convenient reference to tabulated test data with particular assay systems. Another important area of research is the modification of mutagenic activity. In the chapter by Sugimura and Nagao, factors that alter in vitro metabolic activation are discussed, whereas in the chapter by Maher and McCormick, the role of genetic heterogeneity is considered. Equally important is our need to obtain a better understanding of mutation-induction kinetics, and the mathematical analysis by Haynes and Eckardt provides an important step in this direction. An important new approach for the induction of specific locus mutations in human cells in culture is discussed in the chapter by Thilly and co-workers. We have also been concerned about the problems in the practical applica- tion of various short-term tests in commerical testing laboratories. The prob- lems emerging in the evaluation of potentially important new chemicals have been discussed in a chapter by Brusick et al.