Dinitrochlorobenzene is Inherently Mutagenic in the Presence of Trace Mutagenic Contaminants

2,4-Dinitrochlorobenzene (DNCB) is used for immunotherapy of alopecia areata and verruca vulgaris. We initially postulated that the presence of mutagenic contaminants in commercially available DNCB might account for part of its mutagenicity. We have now characterized changes in the dose-mutagenic response curve of 99% DNCB modified by adding 1% concentrations of known contaminants: 1-chloro-2-nitrobenzene; 1,3-dinitrobenzene; and 2,4-dichloronitrobenzene. Dose-response curves were generated using Salmonella typhimurium tester strains TA-98 and TA-100 at concentrations of 0, 1, 5, 10, 25, 50, and 100 μg per plate in a modified Ames assay. We observed a linear dose-response relationship with a slight, but nonsignificant, shift to the right when contaminants were added. We conclude that DNCB is itself mutagenic, and that contaminants play a minor role in its observed mutagenicity.