This report defines the bone marrow-derived elements found in the central nervous system of adult rat radiation chimeras. Four cell types were identified which bore the major histocompatibility (MHC) class I molecules of the donor rat strain thereby indicating a marrow origin. They were: meningeal macrophages, perivascular "microglial" cells, lymphocytes and rare cells with parenchymal microglial morphology. These cells were examined by immunohistochemical methods at the light microscopic and ultrastructural levels. Extended descriptions of the perivascular marrow-derived elements and the parenchymal microglial cells are presented. These latter two cell types, which exist in humans, have a significant role in neuroimmune processes and most probably function as the antigen-presenting cells in the central nervous system of mammals.