The electron and geometric structures of the cis and trans isomers of 3,3′,4,4′-tetrachloroazobenzene (3,3′,4,4′-TCAB) and the trans isomers of 3,3′,5,5′-TCAB and 3,3′-dichloro-4,4′-difluoroazobenzene were calculated by the MNDO method. It was established that the trans isomer, which has a planar structure, is most stable for 3,3′,4,4′-TCAB. Change in the position of the Cl atoms in the azobenzene, i.e., the transition from 3,3′,4,4′-TCAB to 3,3′,5,5′-TCAB, does not lead to appreciable change in the formation energy, the position of the electronic levels, the nature of the frontier orbitals, or the charge distribution in the molecules. This gives reason to suggest that the biological activity of 3,3′,4,4′-TCAB is due to the metabolism products and not to the acti