The in vivo-generator radionuclides ¹⁴⁰Nd (t_1/2 = 3.4 d) and ¹³⁴Ce (t_1/2 = 3.2 d) were used to trace a urokinase-type plasminogen activator (uPA)-targeting mouse monoclonal antibody, ATN-291, in U87 MG xenograft tumor-bearing mice. ATN-291 is known to internalize on the uPA/uPA-receptor pair, making it an appropriate targeting vector for investigating the fate of in vivo generator daughters on internalizing probes. Ante-mortem and post-mortem PET imaging at 120 h post-injection gave no indication of redistribution of the positron emitting daughter nuclides ¹³⁴La and ¹⁴⁰Pr from tumor tissue (p > 0.5). The lack of redistribution indicates that the parent radionuclides ¹³⁴Ce and ¹⁴⁰Nd could be considered as long-lived PET-diagnostic matches to therapeutic radionuclides like ¹⁷⁷Lu, ¹⁶¹Tb and ²²⁵Ac when internalizing bioconjugates are employed.