Thirty-six male C57B1/6 mice were whole-body x-irradiated with 3 Gy to generate lymphocytes with dicentric chromosomes to study the persistence of these lymphocytes in the spleen and peripheral blood to estimate the lifespan of mature B- and T-cells. Blood and spleen were removed from groups of four mice immediately after radiation exposure and on days 1, 3, 7, 14, 28, 56, and 12, thereafter. The initial frequencies of dicentric chromosomes with accompanying fragments observed in splenic T-cells (0.44), splenic B-cells (0.43), and peripheral blood lymphocyte cultures (0.48) initiated on day 0 were not significantly different. For both splenic and peripheral blood-T-lymphocytes, the frequency of cells containing dicentric chromosomes declined in an exponential manner following irradiation, with a 50% reduction in frequency occurring 14 days after exposure. In contrast, the frequency of B-cells containing dicentric chromosomes remained stable through day 7 but then declined precipitously between day 7 and 14 and remained relatively stable, although slightly above baseline, through day 112 post-exposure.