Effects of PTX incubation of hematopoietic progenitor cells on homing. Lethally irradiated hosts received transplants of BM or FL cells, and nonirradiated hosts received transplants of BM cells from neo-resistant donors. Transplants incubated in SCF for 20 hours in the absence or presence of PTX were injected into the hosts. After 24 hours, recovery of CFU-Cs (neo-resistant CFU-Cs in the nonirradiated hosts) from BM, peripheral blood, and spleen was assessed in colony assays. Numbers are given as the percentage of injected CFU-Cs per femur (mean ± SEM), per milliliter blood, and per spleen. ▦ indicates control cells; and ▪, PTX-treated cells. **P < .005. *P < .01 compared with control. (A) Homing of PTX-treated adult BM CFU-Cs to BM was reduced more than 75%. Concomitantly, circulating CFU-Cs were several-fold increased in recipients of PTX-treated cells. (B) BM homing of PTX-treated day-15 FL CFU-Cs also was reduced by more than 75%, with a similar increase of circulating CFU-Cs. (C) In nonirradiated hosts, BM homing of PTX-treated BM CFU-Cs was similarly reduced as in irradiated hosts, and circulating CFU-Cs were similarly increased. In contrast to the irradiated spleens, nonirradiated spleens also showed greatly attenuated homing (> 75% decrease) of PTX-treated donor CFU-Cs from adult BM.