3C10 and 1D9 are two related monoclonal antibodies that specifically identify human mononuclear phagocytes in a large number of sites, including blood monocytes, alveolar macrophages, and macrophages in tissue sections of spleen, lymph node, and skin. The antigen persists on monocytes cultured for greater than 4 wk, but it is not found on giant cells. The 3C10-1D9 determinant is carried by a 55 kD polypeptide, is expressed at approximately 40,000 copies per monocyte, and is protease sensitive. The antigen is clearly different from HLA-class II or Ia-like antigens that have been studied with a new monoclonal 9.3F10. The 9.3F10 antigen is found on B cells, dendritic cells and monocytes; is protease resistant, and occurs on a 33-29 kD doublet typical of class II products. The 3C10 monoclonal provides a clear distinction between human mononuclear phagocytes and dendritic cells. First, monocytes and lymphocytes can be eliminated from plastic-adherent mononuclear cells using 3C10, complement, and two previously described cytotoxic antibodies, BA-1 (anti-B cell) and Leu-1 (anti-T cell). As a result, the trace dendritic cell component of blood can be enriched to considerable purity (65-75%) and yield. Second, immunocytochemical staining of tissue sections reveals that 3C10+ macrophages are anatomically segregated from dendritic cells. Large numbers of 3C10+ cells are found in red pulp of spleen and in regions surrounding lymphatic channels of lymph node. However, 3C10+ macrophages are scarce in white pulp of spleen and the lymphocyte-rich cortex of node that are the sites where dendritic cells are localized. 3C10+ cells in skin are found in the dermis, particularly in leprosy infiltrates, but the Langerhans' cells of epidermis are 3C10-. The distinctive localization of macrophages and dendritic cells is consistent with their respective functions as effector and accessory cells in the immune response.