Semi-empirical molecular modelling calculations of ring rotational energy barriers were carried out for 96 tetrachlorobenzyltoluenes (TCBTs) to provide insight in the possible coplanarity of these compounds, which is required for dioxin-type toxicity. Comparative calculations were performed for the structurally related polychlorinated biphenyls (PCBs) and polychlorinated diphenylethers (PCDEs). On account of this comparison PCBs, PCDEs and TCBTs were divided into groups of different energy barriers. PCDEs and TCBTs were classified into the same relatively high energyebarrier groups. Using the criterion of fitting within a 3 * 10A geometry and using structure activity relationships, our results indicate that two out of the 91 coplanar TCBTs can interact with the dioxin (Ah) receptor. energy of unconstrained molecule; AErOt, difference in energy between the energy minimizations of the unconstrained molecule and the molecule under the constraint of coplanarity; 4, angle of rotation.