An integral transport method was developed and applied to the calculation of the central reactivity worths of various samples measured in a 2700-liter uranium-carbide fast core and in a 4000-liter UO2 fast core. The method was used to obtain corrections to multigroup first-order perturbation calculations, incorporating self-shielding effects inside and outside the sample, as well as scattering, fission, and edge-effect perturbations. Resonance interaction between a 238U sample and the 238U in the core as well as resonance self-shielding in the sample itself were explicitly considered. A similar treatment was used for 235U samples. ENDF/B data were used in all of the analyses. The calculated central worths of 10B and tantalum were in very good agreement with the measured values, indicating that the calculated value of βeff used to convert percent Δk/k to period measurements was reasonably accurate. The calculated worths of 235U were 5 to 10% greater than the measured values. The discrepancy between the calculated and measured values for 238U was 15 to 20%. The discrepancy for graphite in particular and scatterers in general was rather large, indicating that the distribution of the adjoint function is not adequately calculated.