Microglia represent the primary immune effector cells of the adult central nervous system (CNS). The origin of these cells has been a subject of intense debate over the last century. However, immunohistochemical and chimera developmental studies in rodents support the hypothesis that microglia are monocytic in origin. There have been relatively few studies to date on microglia in human fetal development, and the mechanisms by which microglial precursors enter the developing CNS are as yet unknown. It is possible that microglial precursors use combinations of adhesion molecules on cerebral endothelium to gain entry into the developing CNS. In the present study, we have shown the distribution of microglia within human fetal cerebral cortex between 16 and 22 weeks of gestation using RCA-1 lectin histochemistry. We have also demonstrated dual anti-macrophage antibody labelling of these cells in conjunction with adhesion molecules ICAM-1, ICAM-2 and PECAM on cerebral endothelium throughout this period. We conclude that fetal microglia usually occur at highly vascularised sites within the developing human fetal brain and are more specifically associated with the expression of ICAM-2 on cerebral endothelium.