Benzyl chloride, benzyl bromide, p-methylbenzyl chloride, and p-nitrobenzyl chloride were used to study chemical reactivity with 4-(p-nitrobenzyl)-pyridine (NBP), and with guanosine in vitro, in relation to mutagenic potency in S. typhimurium and sister chromatid exchange (SCE) induction in CHO cells. Benzyl bromide was found to be the most reactive compound, followed by p-methylbenzyl chloride, benzyl chloride and p-nitrobenzyl chloride. The order of mutagenicity was p-nitrobenzyl chloride much greater than benzyl bromide greater than benzyl chloride approximately equal to p-methylbenzyl chloride. The compounds tested caused base-pair mutations only. The order of SCE-inducing ability decreased as follows: benzyl bromide greater than benzyl chloride approximately equal to p-nitrobenzyl chloride approximately equal to p-methylbenzyl chloride. The particularly high mutagenicity of p-nitrobenzyl chloride in bacteria may be due to reactions other than direct aralkylation, or it may react particularly actively with DNA. Among the other compounds, benzyl bromide was the most active aralkylating compound, mutagen and SCE inducer. The results suggested that reaction of N2 of guanine, as compared with N-7 of guanine, failed to show any remarkable mutagenicity or SCE induction, since p-methylbenzyl chloride, reacting preferentially at N2 of guanosine, failed to show unexceptional potency.