Concentration-dependent mutagenicity of ICR-191 has been measured in Salmonella typhimurium strain TA98 and in a diploid human cell line. In both cell systems, approximately equigenerational exposure produced mutation linearly related to concentration in the lower range of ICR-191 concentrations tested. Saturation behavior was observed in the human cell assay but not in the bacterial assay. However, a 25-fold greater concentration of ICR-191 was required to induce a significant rise in the mutant fraction in the S. typhimurium assay than in the human cell assay. These differences may be linked to the differences in the biochemical events required for mutation or in the time of exposure to ICR-191.