A comparison of the limitations of the eigenvalues for the effective neutron multiplication factor per neutron generation, k, the multiplication factor per collision, γ, and the fundamental multiplication rate, α, is presented as they concern the neutron spectrum and the spectral properties of the integral transport operators. Numerical examples of analyses of fastneutron plutonium systems are given. Advantages of the rarely used γ eigenvalue are discussed, leading to the main conclusion of this presentation that it should be used more often.