Time-dependent wave-packet-propagation calculations are reported for the isotopic reactions, HD+CH3 and D-2+CH3, in six degrees of freedom and for zero total angular momentum. Initial-state-selected reaction probabilities for different initial rotational-vibrational states are presented in this study. This study shows that excitations of the HD(D-2) enhances the reactivities, whereas the excitations of the CH3 umbrella mode have the opposite effects. This is consistent with the reaction of H-2+CH3. The comparison of these three isotopic reactions also shows the isotopic effects in the initial-state-selected reaction probabilities. The cumulative reaction probabilities (CRPs) are obtained by summing over initial-state-selected reaction probabilities. Theenergy-shift approximation to account for the contribution of degrees of freedom missing in the six dimensionality calculation is employed to obtain approximate full dimensional CRPs. The rate constant comparison shows the H-2+CH3 reaction has the biggest reactivity, then HD+CH3, and D-2+CH3 has the smallest. (C) 2003 American Institute of Physics.