Identification of dendritic cells (DC) is usually done on the basis of their strong MHC class II expression, their typical dendritic morphology and their capacity to induce a strong proliferation of allogeneic T cells. However using these criteria DC can easily be confused with MHC class II positive macrophages (M phi). In addition, the lack of an antibody directed to a specific DC marker greatly hampers the discrimination between DC and M phi. In the present study it is shown that EBM11 (anti-CD68) is a marker specific for both human M phi and DC but in a distinctive way. Human DC locate the EBM11 reactivity in a discrete juxtanuclear spot in contrast to M phi which show EBM11 reactivity throughout the cytoplasm. This greatly improves identification of DC. Light and electron microscopy showed that the CD68 epitope is associated with (phago-)lysosomes. Remarkably the EBM11 spot was only seen in DC after short-term culture, which is an essential step in all classical DC enrichment procedures. Before culture, M phi and DC were indistinguishable. These results show the close relationship between M phi and DC and suggest an important role for the structure of the lysosomal apparatus in these antigen-presenting cells.