Dendritic cells (DC) are specialized leukocytes responsible for inducing antigen-specific immune reactions. Several groups have reported propagating murine DC from lineage depleted phagocytic precursors but reported proliferation rates are low and the specific mechanisms controlling DC maturation have not been clearly elucidated. We define conditions in which DC proliferated in large numbers from murine bone marrow (BM) Scal progenitors, expressed an immature phenotype (MHC IIlow), but increased MHC II expression in response to proinflammatory cytokines. We cultured 2 × 105 purified BM Scal progenitors under standard culture conditions using IMDM with 10% fetal bovine serum, 100 ng recombinant murine (rm) SCF, 20 ng rm IL-3, 50 ng rm M-CSF, 5 ng rm GM-CSF and 25 ng human FLT-3. Under these conditions the cells expanded 19.7 ± 2.6 fold by day 7 (for n= 7 cultures) and 43.9 ± 4.8 fold by day 12. Concurrent with maximum cell proliferation; after 2 weeks in culture, most of the cells were CD80+F4/80+(60.1 + 3.7). Triple analysis showed the cells were also brightly positive for CD86, CD16/CD32, and CD11c. MHC II and CD40 expression were low. These cells were negative for mature dendritic cell markers 33D1 and NLDC-145, and the granulocyte marker Gr-1. The cells were also dendritic like (DLC) morphologically. They exhibited large irregular nuclei, copious organelle-rich cytoplasm, and many cell surface processes. FACS sorted CD80+CD86+ cells were potent allostimulators of T cell proliferation in mixed leukocyte cultures. When we cultured day 12 DLC, for an additional 5 days with 1000 units murine IL-4, the MHC II mean channel fluorescence increased 52.3% ± 26(p<0.001). In addition, when we added 100 units TNF-α, the MHC II mean channel fluorescence increased 9.4% (p<0.001). We have shown that it is possible to generate large numbers of cells with functional, morphological, and immunophenotypical characteristics of immature DC. We have also shown that it is possible to increase MHC II expression with proinflammatory cytokines indicating progression to a more mature cell.