The expression of major histocompatibility complex (MHC) antigen and leukocyte common antigen (LCA) was observed in the amoeboid microglial cells in postnatal rat brain. Considerable MHC class I surface antigen was detected at the plasma membrane and its tubular invaginations in the amoeboid microglia in the corpus callosum using the monoclonal antibody OX-18. In early postnatal (2 and 5 day) rats, the OX-18 positive cells were mostly round but a few possessed stout processes. With increasing age (9 and 15 days) the OX-18 positive amoeboid microglia assumed an oval or elongated form. By the time of weaning (21 days) and in older animals, the immunoreactivity was extremely weak and was detectable only on some branched microglia bearing fine processes. The presence of MHC class Ia antigens with OX-3 and OX-6 was hardly detectable except for a few weakly stained cells in the corpus callosum and cavum septum pellucidum in early postnatal rats. The expression of LCA was observed in amoeboid microglial using the monoclonal antibody OX-1 and this followed a similar temporal pattern to that with OX-18. The additional phenotypic features of amoeboid microglial cells in the present study support their monocytic origin. These cells are endowed with MHC class I antigens which may serve as the restriction elements for T lymphocytes, at least in the early postnatal brain.