As part of a comprehensive program devoted to the integral checks of differential microscopic cross-section data pertinent to the LMFBR program, the experimental and analytical investigations of fast neutron transport in bulk assemblies of iron are described. Time-of-flight measured fast-neutron angular flux spectra at different positions in a simple, clean, homogeneous assembly are analyzed using ENDF/B data as input, MC2 code system, and SN transport calculations. In addition, continuous slowing down theory has been developed to complement precise calculations by indicating direct relationships between cross-section uncertainties and spectra; this approach permits integrating other non-ENDF/B data in the analysis. The well-defined integral experiment permits clear-cut interpretation leading to definitive conclusions with respect to input data and also to checks of standard analytical codes. The Karlsruhe (KEDAK) data set and the ENDF/B-I and ENDF/B-II files for iron have been assessed. On the basis of cross-section uncertainties in iron and of their influence on spectra, it is possible to recommend not only preferred data, but also a redirection in emphasis in differential measurements and evaluation. For fast reactor applications, ENDF/B-I set is prefer able, subject to certain limitations.