In this paper compilation and evaluation of neutron cross sections are oriented in part toward fast spectrum reactor physics. The use of these cross sections with the Sn neutron transport code has resulted in good agreement with experimental results for a variety of critical reactor assemblies. Measurements of critical mass and of the reactivity of a number of isotopes agreed very well with the results of calculations based on evaluated differential cross sections. Systematics of reactivity error magnitudes as a function of flux spectrum indicated several areas for cross-section correction. With these results as a basis for quality evaluation, an examination of the effect of spectral hardness on breeding gain of a fast reactor was made. The highest computed value of breeding gain for large U-Pu metal systems was 1.2 to 1.3, with average 239Pu concentration as low as 2%. Addition of structural material effects on spectral hardness decreased 239Pu breeding gain toward literature values of 0.2 to 0.8.