We have generated two mouse IgM monoclonal antibodies against rat microglia. The RMG-1 antibody recognized a 46-kilodalton cell-membrane antigen of ameboid microglia and cross-reacted with perivascular cells, monocytes/macrophages, and capillary endothelial cells of various organs but not with ramified microglia. In the normal brain, RMG-1-positive cells were found in the choroid plexus, periventricular area, corpus callosum, subarachnoid space and white matter of the cerebrum. The RMG-2 antibody recognized a 78-kilodalton cytoplasmic and cell-membrane antigen of ameboid and ramified microglia and cross-reacted with perivascular cells and monocytes/macrophages of various organs but not with endothelial cells. A small number of RMG-2-positive cells were found in the choroid plexus and subarachnoid space on embryonic day 17. On postnatal day 1, RMG-2-positive cells appeared in the periventricular area and corpus callosum and then migrated to the internal capsule and thalamic nucleus. Thereafter, the number of RMG-2-positive cells increased, reaching a maximum on postnatal day 10 to 14, and gradually decreasing by postnatal day 28. These observations show that microglia are cells of monocytic lineage that enter the brain parenchyma late in embryogenesis or early in the postnatal period and lose at least some monocytic antigens to differentiate into ramified microglia. These monoclonal antibodies will provide useful tools to investigate the relation between microglia, perivascular cells, and endothelial cells in the brain and the kinetics of these cells in normal or pathologic conditions.