Half-wave reduction potential and half-peak oxidation potential have been measured for 11 methyl-substituted benz [c] acridines and unsubstituted benz [c] acridine in acetonitrile with tetraethylammonium perchlorate electrolyte. A linear correlation was obtained between the energy of the lowest unoccupied molecular orbital calculated by LCAO-SCF method and the half-wave reduction potential, and also between the energy of the highest occupied molecular orbital and the half-peak oxidation potential. The correlations between carcinogenic activity and both half-wave oxidation and reduction potentials were interpreted in connection with the electron charge density of the K-region. The results of LCAO-SCF calculation for the electron charge density and the electrophilic superdelocalizability index indicated that the electrophilic reactivity of the K-region or the ring nitrogen atom of methyl-substituted benz [c] acridines may be essential for their carcinogenic activity.