The localization of histocompatibility antigens of the HLA-D locus in dendritic cells (DC) and monocytes (Mo) isolated from human peripheral blood was investigated. Functionally DC were characterized by their capacity to act as strong stimulators in an allogeneic mixed leucocyte reaction. In cytospins, DC were differentiated from Mo by dendritic morphology, strong HLA class II and moderate RFD1 expression on the plasma membrane and acid phosphatase activity in a juxtanuclear spot. Ultrathin cryosections showed that DC had a heavily labelled plasma membrane for HLA-D. In addition, these antigens were found in intracellular vesicles predominantly located in the juxtanuclear zone. This pattern of labelling was not seen in Mo. Obviously, DC concentrate intracellular class II antigens in the same area as lysosomal activity. These results may indicate that this juxtanuclear area is a center of antigen processing in DC.