Frederick J. de Serres, Ph. D. Office of the Associate Director for Genetics National Institute of Environmental Health Sciences Research Triangle Park, North Carolina (U. S. A. ) 27709 The Workshop on Comparative Chemical Mutagenesis was orga- nized to begin the process of problem identification and resolution concerning our needs to evaluate the data on test chemicals arising from assays for mutagenic activity on laboratory organisms. In the past, data on chemical mutagens has been generated and published in the scientific literature on a more or less random basis. Individual chemicals enjoy a brief period of popularity that leads to a burst of publications in the same or sometimes related assay systems. The incompleteness of the data base, in many of these cases, makes comparative mutagenesis difficult or impossible. In our attempts to compare the genetic effects of a given chemical over a wide range of assay systems, we are often interested in making quantitative as well as qualitative compari- sons. To restate the first comparison: is the chemical under ques- tion a weak, moderate or potent mutagen over a wide range of assay systems–or alternatively, does the level of response vary markedly? To make the second comparison, what is needed is information on the spectrum of genetic alterations produced as well as whether this spectrum is consistent over a wide range of organisms.