In order to compare the breeding capabilities of the major nuclear fuels in the spectrum of a fast-breeder reactor, integral measurements have been made for the ratio of their capture and fission cross sections in the third loading of the First Experimental Breeder Reactor (EBR-I, Mark III). The capture-to-fission ratio has been determined as a function of position in the reactor for U233, U235 and Pu239. In addition, for U233 the ratio of (n, 2n) and fission cross sections has been determined. Further, for U238 the following cross-section ratios have been determined: σnγ(U238) / σƒ(U238)(X), where σf(X) refers to the fission cross sections of U233, U235 and Pu239. The capture-to-fission ratio results for the three primary fissile species have been compared with calculations based upon 16-group neutron diffusion theory using two different sets of monoenergetic neutron cross sections, and the agreement is good. The present data show that of the three major fissile species, Pu239 has the highest value of η-1, the maximum number of neutrons available for breeding, for each fissile nucleus consumed.