A new collision probability formulation of integral transport theory is applied in the analysis of small-sample reactivity measurements. The calculational technique features a blending of diffusion theory, used to represent the unperturbed reactor, and integral transport theory, used to represent the sample and that portion of the reactor significantly perturbed by the sample. The theory forms the basis for a perturbation theory computer program, COPERT, which, in conjunction with the one-dimensional diffusion theory program MACH-1, has been applied to the analysis of central reactivity measurements in the ZPR-3 fast critical assembly. The method provides a convenient means of accounting for sample-size effects.