In human tissues different populations of dendritic cells (DC) emerge from hematopoietic progenitor cells (HPC) in the bone marrow, with the intermediate steps of differentiation not being completely understood. In vitro, DC can be directly obtained from HPC or from blood monocytes (MO) cultured in the presence of GM-CSF and additional cytokines. We compared the antigenic profile of DC derived from either MO or HPC and studied their capacity to stimulate naive lymphocytes (LY) in the allogeneic mixed lymphocyte reaction. Both types of DC expressed high levels of CD1a, MHC class II, CD80, CD86 and CD40 and were potent stimulators of LY proliferation. DC of HPC origin, though, induced a stronger mixed lymphocyte reaction than MO-derived DC and showed a slightly higher average expression of costimulatory antigens. Low-level expression of CD14 did not negatively correlate with DC function on DC stimulated with lipopolysaccharide and was even slightly higher expressed on DC differentiating from HPC than on DC from CD14+ MO.