Nanosecond bursts of monoenergetic neutrons in the 1 Mev range are injected into various size assemblies of iron. The flux in these assemblies is observed to decay exponentially with characteristic nanosecond decay constants (λ). λ is shown to be composed of a sum of terms which represent loss of neutrons by leakage and through energy degradation by both nonelastic and elastic scattering. The sum of these two last effects can be represented by a total removal cross section which can be determined by measuring λ as a function of assembly size. A theoretical development is given for calculating the contribution to this total cross section due to elastic scattering; hence the total nonelastic cross section can be determined. Nonelastic cross sections for iron have been measured by this technique in the range of primary neutron energies 0.8–1.5 Mev.