A method has been developed for predicting the interaction effect on control-rod worth and applied to a fast reactor system. The method is based on the exact perturbation theory to which several assumptions have been applied. These assumptions—numerically confirmed to be valid—all relate to separability of the effects on flux change brought by different factors of perturbation. The method permits prediction of the worth of a combination of any number of control rods solely from information obtained on single-rod worth calculations. Adoption of this method reduces from 2N to N+L the number of calculations necessary for N rods of interest. Comparison with results obtained by the conventional direct k-difference method has revealed that the present method predicts the worth of a combination of control rods to within a few percent—1.3% at most in seven numerical examples studied, covering both symmetric and a symmetric rod configurations and with the interaction effect ranging about—10—+ 15%.