Cytokine combinations were tested for their ability to expand murine bone marrow (BM) progenitors in short-term suspension cultures (Δ-cultures) with the aim of providing an enriched source of progenitors for BM transplantation (BMT). In a comparison of the efficacy of the combinations interleukin-1 (IL- 1) + IL-3, IL-1 + kit-ligand (KL), and IL-1 + IL-6 + KL, BMT with IL-1 + KL expanded progenitors was found to be most effective in accelerating the recovery of peripheral blood leukocytes, platelets, and erythrocytes in lethally irradiated mice. The ex vivo expansion of BM in IL-1 + KL-stimulated Δ-cultures also greatly reduced the number of transplanted cells needed to provide radioprotection. All mice survived at least 30 days when receiving 5 x 103 Δ-cultured d1 5-fluorouracil (5-FU) BM cells (BM cells harvested 1 day after 5-FU administration), whereas complete survival of mice receiving fresh d1 5-FU BM required BMT with a 200-fold greater number of cells, BMT with expanded BM lead to predominantly donor-derived hematopoietic reconstitution for 280 days postprimary BMT and another 71 days after secondary BMT. The expansion of BM did not adversely effect the proliferative capacity and lineage potential of the stem cell compartment.