HPLC/MS single ion monitoring methods have been evaluated for the determination of trace levels of 4- hydrazino benzoic acid (4-HBA) in drug substances. These 4-HBA have been highlighted as potential genotoxins. HPLC/MS was found to be more promising and limits of quantification was 0.52 ppm. For one drug substances excellent recoveries of 104.6% were obtained at the 0.5 ppm level.