In this article, we address the issue of persistence fit to the combined dicentric, acentric fragment, and of chromosome exchanges following acute in vitro translocation frequencies, which showed that all exposure of rat peripheral blood to 137 Cs. Irradia-three aberration types declined at equivalent rates. tion occurred 24 hr after culture initiation, and meta-The frequencies of dicentrics and fragments dephase chromosomes were prepared 2, 3, 4, and 5 clined to a plateau of zero, while translocations days later. Chromosomes 1, 2, and 4 were painted reached a plateau at frequencies significantly in unique colors and scored for structural aberra-greater than zero. The decline in translocations with tions. Dicentric chromosomes and acentric frag-time is inconsistent with prevailing theoretical exments diminished rapidly with time, as expected. pectations, but is consistent with a model where Translocations exhibited greater persistence, but some translocations are fully stable (persistent) and still showed a reduction in frequency, reaching a some are unstable (not persistent) through cell diviplateau of approximately 65 and 55% of their initial sion. These results may have implications for radiavalues, 4 days after exposure to 1 and 2 Gy, re-tion biodosimetry in humans.