Reduced homing capacity of HPCs with donor and recipient age. Lethally irradiated young or old Ly-5.1 mice were injected with 2 × 10⁶ old or young Ly-5.2 donor cells. Three hours later, HPCs that had homed to the BM (□) or spleen (▪) were measured by hematopoietic colony formation in vitro. Shown are the mean (± SEM) percentages of donor-derived HPCs recovered per organ, relative to numbers injected. Data are pooled from 3 independent experiments with 3 to 5 mice per group. Differences between young cells injected into young recipients and either young cells transplanted into old recipients or old cells transplanted into young recipients are significant (P < .05).