A one-group method for the calculation of neutron balances in fast reactor cores is developed and evaluated. The key feature of the method is the definition of two spectrum characterization parameters in terms of spectrum-averaged one-group cross sections for the homogenized core composition: where ξel is the mean logarithmic energy decrement for elastic moderation and ∑f, ∑TR, and are fission, transport, and removal cross sections, respectively. All required cross sections can then be correlated in the form = σ1 Sg (where and g are constants; one pair of values correlated for each cross section) except for threshold fission for which = σ1Rg. A rapidly converging iterative procedure is presented through which S and R can be determined for any core composition. Microscopic cross-section data are correlated in the above form using the 26-group ABBN multigroup set as parent data. The one-group method is tested for 45 different fast reactor core compositions by comparing the results of the one-group calculations with those of 26-group calculations. The results are found to agree within an average error of ±1.77% in the material buckling or to ±0.69% in effective multiplication factor. One-group relationships are also developed for the calculation ofprompt-neutron lifetime, Doppler reactivity, and other parameters of interest.