The Fukui function is one of the most important concepts in the theory of chemical reactivity. For the fixed positions of nuclei, it describes reorganization in electron density of a given molecule due to overall chemical oxidation/reduction. The concept of the Fukui function was extended to reactive systems, giving a real two-reactant description of the chemical reaction. In this paper, the finite-difference approach was adopted to derive formulas for diagonal and off-diagonal Fukui functions in local and in atoms-in-molecule resolutions. The obtained results generalize the approach of Parr and Yang or Yang and Mortier into reactive systems, e.g. interacting molecules. The self-consistent charge and configuration method for subsystems (SCCCMS) was applied to calculate these quantities. A combination of Becke＇s three-parameter hybrid functional and the Perdew/Wang 91 gradient-corrected functional with the 6-31G* basis set was used in Kohn-Sham calculations. Methyl radical addition to ethylene was taken as an illustrative example. Charge transfer from ethylene to the radical (forward donation), as well as that from the radical to ethylene (backward donation), was considered. We have shown that off-diagonal Fukui functions should be taken into account in realistic models of interacting molecules.