Regenerating detectors incorporate fertile material along with fissile material to sustain the sensitivity of neutron monitors located in high-flux environments. A mathematical model for optimizing the initial composition of the regenerating material was formulated and programed. An experimental evaluation, based on mass-spectrometer determinations of the change in isotopic composition as a function of neutron exposure, was completed using plutonium isotopes. The agreement between experiment and calculations is within uncertainty introduced by basic cross-section data.