The effect of mono-, di-, tri-, and tetra-chlorinated benzene on mitochondrial oxidative phosphorylation was determined and the following results were obtained. 1) Chlorinated mono-aromatic hydrocarbons inhibited respiratory control strongly by increasing state 4 respiration and decreasing state 3 respiration. 2) Lower concentrations of mitochondrial protein showed higher inhibitory effects for 1,2,3-tichloro benzene on respiratory control. 3) The respiratory control index decreased with increase in the number of chlorine. 4) The respiratory control index decreased with increase in the concentration of chlorinated benzene. 5) Decrease in respiratory control index was in the order of orth>meta>para-dichloro benzenes. There were differences between the isomers.