The effect that interference between resolved resonances has on averaging multigroup cross sections is examined for thermal reactor-type problems. A simple and efficient numerical scheme is presented to correct a preprocessed multigroup library for interference effects. The procedure is implemented in a “design-oriented” lattice physics computer code and compared with rigorous numerical calculations. The approximate method for computing resonance interference correction factors is applied to obtaining fine group cross sections for a homogeneous uranium-plutonium mixture and a uranium oxide lattice. It was found that some fine group cross sections are changed by more than 40% due to resonance interference. The change in resonance interference correction factors due to burnup of a pressurized water reactor (PWR) fuel pin is examined and found to be small. The effect of resolved resonance interference on collapsed broad group cross sections for thermal reactor calculations is discussed. It is found that the 238U and 235U epithermal cross sections are fairly insensitive to interference effects, but the 239Pu value increases ∼3.5%, and the 240Pu value decreases by more than 7% for a PWR pin.