Experimental determinations of capture rates of structural materials used in fast breeder reactors, such as iron, chromium, nickel, and stainless steel, normalized to the 235U fission rate, were performed. The aim of these experiments was to check structural material multigroup cross-section libraries in the 1- to 100-keV range, where substantial discrepancies among various cross-section evaluations are not yet resolved. The experiments were carried out in an ARGONAUT-type RB-2 reactor, using the Null Reactivity Oscillation method, on test media composed of quasi-homogeneous loose particle mixtures. Comparisons were carried out with corresponding calculated values, showing a trend of these values to overestimate the measured quantities. Furthermore, evaluations of and of of 235U were made in the same intermediate spectrum. These results are not in disagreement with the indications of recent 235U cross-section measurements as far as the σc/σf of 235U is concerned.