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.