To assess the cytogenesis and the structure of the microglial cells, we studied mesencephalons in 47 human fetuses at 7th-40th week of gestational age, and in 18 adult brains from 20 to 70 years. The microglial cells were identified and characterized by morphological criteria using immunohistochemical and histochemical techniques. As early as in the 8th week of gestational age RCA-1 positive cells were detected, mainly in form of amoeboid microglial cells. These microglial cells were observed around the germinal matrix, and at or near the wall of blood vessels. RCA-1 positive cells which were detected within leptomeninges were large but without processes. At the 16th-40th week of gestational age we observed in mesencephalon, amoeboid microglial cells, and also RCA-1 positive and ferritin reactive ramified microglial cells.