High-purity Cu specimens doped by electron irradiation with interstitial clusters and single vacancies were annealed isochronally through the temperature range of stage II-1V. After each step of the annealing program, the doped samples received an additional small test irradiation at 4 K followed by an anneal at 65 K. By measuring the fraction,P, of the test interstitials which survived this anneal, reactions of the mobile test interstitials with the doping defects were studied. A substantial increase ofPwas observed as the doping defects were annealed through stage III. A similar increase was found in the 70 K damage production. These increases are attributed to the clustering of doping vacancies in stage HI and can, therefore, be taken as evidence for vacancy migration in this stage.