The monoclonal antibody ER-MP20 specifically reacts with mouse macrophage precursor cells in the mid-stage of their development (late CFU-M, monoblasts and monocytes). The antigen is a 14 kD surface protein which is very similar to Ly-6C and may be analogous to human CD59. It is inducible by IFN-alpha, IFN-beta and IFN-gamma. In tissue sections, the antigen is found on macrophage precursor subpopulations. In the bone marrow and hemopoietic islands of the lymphoid organs, and in the spleen. Activated macrophages in inflammatory tissues also express the ER-MP20-related antigen.The monoclonal antibody ER-MP20 has been raised after immunization of rats with mouse macrophage cell lines and reacts with mouse macrophage precursor cells. The monoclonal antibody also identifies activated macrophages in inflammatory tissues where the simultaneous use of the murine pan-macrophage marker BM8 (anti-F4/80) is recommended. In combination with an anti-mouse CD31/PECAM-1 antibody, ER-MP20 can be used to evaluate the cellular composition in murine bone marrow (e.g. using flow cytometric analysis). ER-MP20 also detects a wide range of endothelial cells.