Spectral synthesis, the representation of the energy dependence of the neutron flux by a combination of overlapping trial modes, has received only limited application to the analysis of fast reactors, because the savings in computational effort achieved via synthesis have not been sufficiently dramatic to compensate for the. occasional disconcerting failures of the methods. This Note develops an approximation procedure combining spectral synthesis with Wie-landVs method which recovers the large savings in computation time expected with synthesis methods. Such a scheme is illustrated by applying it to calculate the flux in a fast critical assembly.