Histocompatibility antigens of the HLA D locus on the surface of human dendritic cells (DC) were visualized in the electron microscope using immunogold labelling. DC from peripheral blood expressed DR that was frequently concentrated at junctions between aggregating DC and lymphocytes or DC and macrophages. Labelling with an antibody to DQ was more diffuse and was not concentrated at points of cell-cell contact. The D locus antibody RFD1 labelled DC in distinct patches that were sometimes located at points of cell contact. Upon labelling DC with antibody to DR and incubating the cells at 37 degrees, some label remained on the cell surface but some was found in deep channels which appeared to be formed between veils at the surface of the cell and became internalized in membrane-bound structures. Under the same conditions, gold bound to DQ molecules remained on the surface of DC. Gold labelling RFD1 also remained mainly on the cell surface but there was occasionally internalization of patches into the cells through depressions in the cell membrane. The changes in distribution of the label on warming the cells suggests that materials bound to different D locus products may be 'processed' differently.