Dendritic cells (DC) are the most effective antigen-presenting cells. Many studies now show that DC can be generated in vitro from a number of starting cell populations containing hematopoietic precursors. The protocols used involve different combinations of cytokines including granulocyte macrophage-colony stimulating factor (GM-CSF), which supports myeloid precursors, or interleukin-7, which supports lymphoid precursors. DC are commonly generated by in vitro culture of bone marrow or monocytes with GM-CSF and other cytokines. However, these cultures do not sustain DC production for long periods of time and do not allow the identification or study of intermediate stages in cell development. In vitro cytoldne-dependent cultures of DC precursors do provide a reliable source of DC for stimulating immune responses. However, use of cells produced in cytokine-dependent cultures for the study of DC differentiation is limited, as DC development in vivo differs in cytokine dependency.