Molecular Biology and Biophysics Programme, Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur-721 302 Metabolism and cancer-inducing ability of methylphenanthrenes are determined using quantum mechanical method in the MINDO/3 approximations of Bingham et al. Several reactivity and molecular parameters are used for the purpose. Results of the calculations indicate that none of the isomeric methylphenanthrenes are active as cancer initiator and that 1,2-dio1-3,4-epoxide which is the ultimate metabolite for each of them is very weak. The predictions are in striking agreement with the experiment.