The mutagenicity of 24 benzyl derivatives, containing a variety of substituents and leaving groups, were assayed in strain TA100 using the Ames plate-incorporation assay. p-Nitrobenzyl chloride (12 000 revertants/mumole), p-nitrobenzyl tosylate (6100 revertants/mumole), and p-acetoxybenzyl chloride (100 revertants/mumole) were mutagenic; none of the remaining 21 compounds were mutagenic. p-Nitrobenzyl chloride was also found to be mutagenic in strain TA98 (700 revertants/mumole), but not in strain TA98NR (a strain deficient in nitro reductase activity). p-Acetoxybenzyl chloride was nonenzymatically hydrolyzed to p-hydroxybenzyl alcohol and p-acetoxybenzyl alcohol. These findings suggest that nitrobenzyl derivatives were mutagenic due to nitro reductive metabolism and that p-acetoxybenzyl chloride was mutagenic due to the intermediate formation of p-hydroxybenzyl chloride during the hydrolysis of p-acetoxybenzyl chloride.