Antigen-presenting cells (APCs) that show the expression of major histocompatibility complex (MHC) class II molecules in adult persons are related to an early phase of immunological response. These molecules are responsible for the binding, transport, and presentation of a foreign antigen to helper T lymphocytes and determine the type of antibodies produced. They also stimulate the multiplication of specific B lymphocytes and participate in the elimination of autoreactive lymphocytes and maturation of T lymphocytes. Cells with MHC II molecules expressed on their surface were observed in the frontal and temporal lobes of 30 brains of human fetuses with normal development between gestation weeks (GW) 11 and 22. MHC II expression was noted during the whole interval under study. Its immunocytochemical localization was noted on the surface of the cerebral meninges cells, in the choroid plexus of the lateral ventricle and blood vessel lumen, and in ameboid microglia (AM) and ramified microglia (RM) cells in both cerebral lobes of the human fetus. The expression of MHC II that occurred on the cells of the central nervous system (CNS) already in GW 11 may be evidence not only of an early capability of immunological protection of the fetal nervous system, but also of a significant role potentially played by this system in normal embryogenesis. Despite continuous controversy over the cellular lineage of microglia origin, the expression of MHC II on AM and RM cells indicates its mesodermal origin, as in other APCs.