Dendritic cells (DC) are professional antigen-presenting cells. While DC have been derived in vitro from circulating monocytes they can also be directly generated in vitro from bone marrow and peripheral blood hematopoietic progenitor cells (HPC). In the current study we compared the effectiveness of various cytokine combinations to induce both proliferation and differentiation of murine bone marrow HPC into dendritic like cells. Sca 1+progenitor cells were purified by immunoaffinity selection from the bone marrow of adult C57BL/6 mice. Under standard culture conditions (IMDM, 10% fetal bovine serum, 100 ng/ml mSCF, 20 ng/ml mIL-3 and 10 ng/ml mM CSF) the number of Sca 1 + cells seeded initially affected proliferation but not the differentiation pattern. The optimum was an initial cell concentration of 2.5× 105 cells/ml. Under these conditions the cells expanded 10 ± 1.3 fold by day 7, at which time 40% of the cells expressed the macrophage antigen F4/80 and, 9% and 8% of the cells expressed the dendritic cell antigens CD 80 and CD 86, respectively. By day 11 - 14 the cell number increased an additional 1.8 ± 0.3 fold and the surface marker expression was 45% F4/80, 37% CD 80 and 30% CD 86. Morphologically these cells expressed veiled cytoplasmic processes characteristic of DC. Experiments were performed in which the mM-CSF was increased to 50 ng/ml and 5 ng/ml of mGM-CSF was also added to the standard culture medium. Under these conditions the cells expanded 17 fold by day 7 and by day 14 the cells expressed the following phenotype: 56% F4/80, 64% CD 80 and 22% CD 86. Further addition of 25 ng/ml hFLT3 ligand to these culture conditions enhanced proliferation another 20% and resulted in increased expression of all cellular monocyte/dendritic surface markers (65% F4/80, 70% CD 80 and 34% CD 86) by day 14. We conclude that optimum conditions for enhancing proliferation and differentiation of dendritic cells from Sca 1 + progenitors require a combination of myeloid and stromal growth factors which include FLT3 ligand.