AbstractInop/opmice, immunohistochemical and electron microscopic techniques were used to examine the effects of theOPmutation on dendritic cell populations in lymphoid tissues and skin. In the thymic medulla, T cell zone of lymph nodes, and splenic white pulp ofop/opmice, numbers of NLDC‐145‐positive dendritic cells were not decreased. Compared to the normal litter‐ mates, numbers of BM8‐positive macrophages were reduced in various tissues of the mutant mice, including the lymphoid tissues. These dendritic cells ofop/opmice expressed la antigens but not F4/80 and BM8 antigens. Ultrastructurally, the dendritic cells developed a tubulo‐ vesicular system typical of interdigitating cells, but they were abnormal in that interdigitation of their cytoplasmic processes was not prominent. In the epidermis of theop/opmice, dendritic cells expressed NLDC‐145, F4/80, la antigens, and adenosine diphosphatase or adenosine triphosphatase activity, and numbers of NLDC‐145‐, la‐, or ADPase‐positive dendritic cells were reduced slightly, but these reductions were not significant statistically. Bir‐ beck granules were detected in most of them electron microscopically. These results indicate that nonlymphoid dendritic cells develop in the lymphoid tissues and skin ofop/opmouse, suggesting that they are differentiated from granulocyte‐macrophage colony‐forming cells or earlier hematopoietic cell precursors.