Benzidine (BZ) and its six structural analogues (2-aminobiphenyl [2-ABP], 4-aminobiphenyl [4-ABP], 3,3'-diaminobenzidine [DABZ], 3,3'-dichlorobenzidine [DCBZ] 3,3'-dimethoxybenzidine [DEBZ], and 3,3'-dimthylbenzidine [DMBZ]) were examined for DNA damage in human lymphocytes using the alkaline comet assay. All the tested compounds showed a distinct disparity in their respective DNA-damaging capacities with an order of DABZ > BZ > DCBZ > 2-ABP > DEBZ > 4-ABP > DMBZ when lymphocytes were exposed to these chemicals for 2 h. Results show that the DNA-damaging effects of these compounds had no bearing on some physicochemical parameters including oxidation potentials, the energy differences between the lowest unoccupied molecular orbital and the highest occupied molecular orbital, ionization potentials, dipole moment, and relative partition coefficient. On the other hand, the free radical scavengers, including catalase, SOD, BHT, EDTA, and histidine exerted varying degrees of inhibitory effects on the DNA damage caused by benzidine. This suggests that genotoxicity in lymphocytes caused by benzidine proceeded via a reactive oxygen species (ROS)-mediated mechanism.