Brains from human fetuses of 13 to 27 weeks gestation have been examined immunohistologically for the presence of macrophages using the marker alpha-1-anti-chymotrypsin. A preliminary study demonstrated this to be a satisfactory marker of brain macrophages, although macrophages were also weakly positive for the more specific marker MAC-387. Macrophages were widely present within the cerebral hemispheres following their rapid accumulation between 14 and 16 weeks of gestation. They were identified in characteristic locations which, in the earliest gestation brains examined at 13 weeks, included the mid-line of the corpus callosum, around the optic tract and at the junction of the external and internal capsules near the apex of the putamen. Subsequently, macrophages were identified in abundance in the internal and external capsules and, by 22 weeks gestation, in the periventricular tissues. Their consistent presence and distribution indicate that at least the majority of these macrophages are a normal feature of the developing brain possibly related to remodelling processes.