The energy distribution of neutrons, from the source energy of 14 MeV down to the (n,2n) threshold, can be approximated by effective one-group cross sections for such high and medium mass number elements as lead, copper, zirconium, and iron. The same is true for 238U, when the fast fission factor ϵ is applied in a special manner to account for the added multiplication below the (n,2n) threshold. Two groups are required to obtain a reasonable description of the amplification in beryllium.