The ratio βeff/l has been deduced from reactor noise measurements in several uniform light-water lattices in the Plutonium Recycle Critical Facility. These lattices included one in which the fissile material was slightly enriched uranium, one in which the fissile material was plutonium, and five lattices in which the fissile material contained both uranium and plutonium. These measurements supply a set of experimental data over a range of plutonium enrichments that are applicable to plutonium recycle situations in thermal reactors. The measured values of βeff/l range from 33 ± 3 sec−1 for a lattice of Al-Pu rods to 153 ± 8 sec−1 for a lattice of UO2 rods. Calculated values of βeff/l, obtained with a straight-forward reactor design calculational method, are in good agreement with measured values.