Monoclonal antibodies (MAbs) to functionally heterogeneous populations of human rheumatoid arthritis (RA) synovial tissue macrophages and lipopolysaccharide (LPS)-activated U937 cells were generated. These MAbs were used to characterize macrophages in situ in the synovial pannus and to study relative antigen expression on the surface of cells isolated from the synovium and from normal peripheral blood. Monoclonal antibody 3D8, an anti-CD13 MAb, reacts with an antigen expressed on the surface of blood monocytes and is a monocyte activation-related antigen that is upregulated by exposure of monocytes to interferon-gamma (IFN-gamma) and LPS. The expression of the 3D8 antigen increases in parallel with MHC class II antigen expression and also is upregulated in culture as monocytes mature to macrophages. 3D8 antigen is expressed strongly on RA synovial tissue lining cells, which are thought to be composed of macrophages. 8D7 antigen expression, detected by MAb 8D7, increases on blood monocytes on cellular activation with LPS and interferon-gamma, but in contrast to the 3D8 antigen, does not increase with monocyte maturation in vitro. The 8D7 antigen is expressed differentially on density-defined macrophage subpopulations isolated from RA synovial tissue and is expressed more strongly on macrophages that are nonangiogenic than those that are angiogenic.