A lumped (point) representation of the reactivity of a mixed-assembly reactor is derived from the basis of perturbation theory. This gives good agreement with exact static reactivity calculations for some simple examples. It is also compared with the simple partial reactivity model used widely in fuel management theory. A similar comparison is made for alternative representations in terms of the excess multiplication factor of the system. Although it is shown that the error in using the partial reactivity concept may be regarded as second order, the transient behavior of three simple refueling systems predicted by the point reactivity model differs markedly from previously published partial reactivity results.