Pertuzumab (Omnitarg) is a novel antibody against HER-2, domain II. HER-2 is a tyrosine kinase receptor that is overexpressed in several carcinomas, especially breast cancer. Pertuzumab, labeled with the low-energy beta emitter (177)Lu, might be a candidate for targeted radiotherapy of disseminated HER-2-positive micrometastases. The radiolabeled antibody [(177)Lu]pertuzumab showed favorable targeting properties in BALB/c (nu/nu) mice with HER-2-overexpressing xenografts. The absorbed dose in tumors was more than five times higher than the absorbed dose in blood and more than seven times the absorbed dose in any other normal organ. Experimental therapy showed that [(177)Lu]pertuzumab delayed tumor progression compared with controls (no treatment, P < 0.0001; nonlabeled pertuzumab antibody, P < 0.0001; and (177)Lu-labeled irrelevant antibody, P < 0.01). No adverse side effects of the treatment could be detected. Thus, the experimental results support the planning of clinical studies applying [(177)Lu]pertuzumab for therapy.