The cobblestone-area forming cell (CAFC) assay permits a direct measurement of the seeding of primitive and more mature murine hematopoietic stem cell subsets by comparing the number of CAFC in the original transplant with the number of CAFC retrieved from bone marrow (BM) and spleen after transplantation. We found no differences in seeding efficiency between the more mature and primitive CAFC subsets, nor between seeding efficiencies of stem cells from low-density (LD) fractions of normal and day-6 post-5-fluorouracil BM. The data show that 18% to 20% of all intravenously transplanted stem cell subsets seed to the BM, whereas 8% to 10% seed to the spleen. In addition, similar seeding efficiencies were found for day-12 spleen colony-forming unit (CFU-S-12) as was determined by retransplantation. Previously, it has been reported that a 2- to 3-hour preincubation of BM with interleukin-3 (IL-3) enhances the in vivo repopulating ability of a graft. To test whether hematopoietic growth factors affected this increased engraftment by enhancing the seeding of the transplanted marrow, we assessed the 16- to 18-hour seeding efficiency of short- and long-term in vivo repopulating stem cell subsets to BM and spleen using the CAFC assay, after preincubation with or without hematopoietic growth factors. A 2- to 3-hour preincubation with IL-3, or a combination of IL-3, IL-12, and steel factor, at 37 degrees C, led to a substantial decrease in seeding compared with control (which was kept on ice) of all hematopoietic subsets measured, both in spleen and BM. In concert with these data, the long-term in vivo repopulating ability of growth-factor incubated BM was also decreased when compared with control. In conclusion, we have been unable to observe a beneficial effect of growth factor preincubation on the repopulating ability of a graft.