Human CD34+ bone marrow progenitors cultured in the presence of granulocyte-macrophage CSF (GM-CSF) develop along a myeloid pathway, and the addition of exogenous TNF-alpha leads to the differentiation of dendritic cells among the myeloid progeny. These bone marrow CD34+ -derived dendritic cell that develop during 2-wk culture have the same morphologic, phenotypic, and functional properties that distinguish mature dendritic cells in blood. c-kit ligand does not directly influence dendritic cell differentiation per se, but rather increases the total cell number in synergistic combination with GM-CSF and TNF-alpha. This degree of expansion translates into an effective yield of approximately 1.7 x 10(6) mature dendritic cells per single ml of normal adult human bone marrow, compared with approximately 10(6) dendritic cells usually obtained from 450 to 500 ml of peripheral blood. In addition to dendritic cells that constitute approximately 10 to 15% of the total myeloid progeny, the cultures contain monocytes/macrophages and intermediate granulocytic precursors. Monocytes/macrophages and dendritic cells together comprise all of the class II MHC-positive progeny. Sorted cells bearing the CD14+ HLA-DR+ phenotype of mature monocytes are at least 1.5 to 2 logs less active than CD14- HLA-DR+ dendritic cells as stimulators in the allogeneic MLR, even though both CD14+ and CD14- subpopulations share expression of several costimulatory ligands. The synergistic combination of c-kit ligand, GM-CSF, and TNF-alpha therefore expands substantial numbers of immunostimulatory CD14- HLA-DR+ dendritic cells from defined CD34+ progenitors in human bone marrow. This should facilitate the use of dendritic cells in the manipulation of T cell-mediated immune responses.