Microglia, the resident CNS macrophages, represent about 10% of the adult brain cell population. Although described a long time ago, their origin and developmental lineage is still debated. While del Rio-Hortega suggested that microglia originate from meningeal macrophages penetrating the brain during embryonic development, many authors claim that brain parenchymal microglia derive from circulating blood monocytes originating from bone marrow. We have previously reported that the late embryonic and adult mouse brain parenchyma contains potential microglial progenitors [F. Alliot, E. Lecain, B. Grima, B. Pessac, Microglial progenitors with a high proliferative capacity in the embryonic and the adult mouse brain, Proc. Natl. Acad. Sci. U.S.A. 88 (1991) 1541-1545]. We now report that they can be detected in the brain rudiment from embryonic day 8, after their appearance in the yolk sac and that their number increases until late gestation. We also show that microglia appear during embryonic development and that their number increases steadily during the first two postnatal weeks, when about 95% of microglia are born. Finally, the main finding of this study is that microglia is the result of in situ proliferation, as shown by the high proportion of parenchymal microglial cells that express PCNA, a marker of cell multiplication, in embryonic and postnatal brain. Taken together, our data support the hypothesis that terminally differentiated brain parenchymal microglia are derived from cells originating from the yolk sac whose progeny actively proliferates in situ during development.