Murine leukemia virus mutants ts3 (Moloney) and ts24 (Rauscher) both formed late-budding structures on the cell membrane at restrictive temperature. They both accumulated core polyproteins Pr65gag and Pr180gag-pol in cell membranes, but the envelope precursor was rapidly turned over. After shift to permissive temperature in the presence of cycloheximide, the accumulated precursors were sequentially cleaved via discrete intermediates both during the final stages of the budding process and in newly released virions to yield the finished virion core proteins and reverse transcriptase. The precursor form of reverse transcriptase was not enzymatically active and became activated partially or entirely inside released virions.