“Competitive homing” of PTX-treated and control BM CFU-Cs. Lethally irradiated recipients were injected with a 1:1 mix of control (SCF only) and PTX-treated (SCF + PTX) BM cells. As an additional control, some recipients were injected with SCF-treated cells alone. After 24 hours, homing of control CFU-Cs injected alone, of control CFU-Cs coinjected with companion PTX-treated cells, and of PTX-treated CFU-Cs coinjected with companion control cells to BM, peripheral blood, and spleen was assessed by colony assays. Error bars indicate SEM. As in the previous experiments, even when PTX-treated CFU-Cs were coinjected with companion control BM cells, their homing to BM was significantly reduced, with a concomitant increase in circulating CFU-Cs, in comparison with control CFU-Cs coinjected with PTX-treated cells or injected alone. Splenic homing was not affected. At the same time, the companion PTX-treated cells had no effect on the homing of coinjected control cells, since there was no difference in homing between control cells injected alone and control cells coinjected with companion PTX-treated cells, evidencing the hematopoietic cell-intrinsic nature of the PTX effect (control cells injected alone, □; control cells injected with companion PTX-treated cells, ▦; PTX-treated cells injected with companion control cells, ▪). *P < .05. **P < .01.