An anti-human macrophage monoclonal antibody, AM-3K, was produced using human alveolar macrophages as antigen. The molecular weights of the antigen recognized by AM-3K were 120 and 70 kD. Immunohistochemically, AM-3K reacted intensely with most macrophages in lymphoreticular organs and in many other organs and tissues. In the spleen, AM-3K reacted with red pulp macrophages, some white pulp macrophages, and tingible body macrophages in lymphoid follicles. In the lymph nodes, many macrophages distributed in the outer cortex, paracortical area, medulla, capsule, or within lymphoid follicles showed an intense reaction for AM-3K. Kupffer cells of the liver, macrophages in the connective tissues, and interstitial macrophages of the kidneys, pancreas, testis, and many other organs were also strongly labelled. AM-3K also reacted with macrophages in many pathological conditions. This antibody, however, did not react with dendritic cell populations, such as epidermal Langerhans cells, interdigitating cells in the paracortex of the lymph nodes, and follicular dendritic cells within the lymphoid follicles, nor with cells other than macrophages, including epithelial cells, vascular endothelial cells, lymphocytes, and granulocytes. Reaction products for AM-3K were found on the cytoplasmic membrane of macrophages by immunoelectron microscopy. In both cryostat sections and formalin-fixed paraffin sections, this monoclonal antibody recognized the antigen present on the cell surface membrane of tissue macrophages, but not monocytes or dendritic cells.