Linearity of the homing assay. Lethally irradiated mice were injected with 10⁶ to 1.2 × 10⁸PKH26-stained BM cells. Three hours later, BM (○) and spleen (•) cells were analyzed by flow cytometry. (A) The percent of labeled cells in each organ is shown for 32 individual mice from 18 experiments. Correlation coefficients >0.9 demonstrate a linear relationship between hematopoietic cell homing and transplanted cell dose. (B) The values in A were multiplied by organ cellularity in each case to calculate the fraction of cells homing to BM or spleen relative to the number transplanted. The essentially flat scatter of points over the x-axis demonstrates that hematopoietic cell homing is largely independent of graft size over the range of cell doses normally transplanted.