Macrophages were found in the meningeal sheath of the human optic nerve at all ages from 8 to 18 weeks post-conception. At 8 weeks the majority of macrophages contained few cytoplasmic organelles or vacuoles, but even at this age a small number of cells packed with small dense bodies were present. With increasing age the number of organelles increased and some vacuolated macrophages were present. The morphology of macrophages largely depended on the part of the meninges in which they were situated. Those lying in the subarachnoid space or loose outer layers of the dura were irregularly shaped and often vacuolated, whereas those lying in the tightly packed layer of arachnoid at its junction with the dura were elongated and contained few, if any, vacuoles. A few meningeal macrophages were observed apparently migrating along the fibrous septa which carry blood vessels into the substance of the nerve. The main structural differences between meningeal macrophages and optic nerve microglia (Sturrock, 1984) were the presence in the latter of numerous small vacuoles and long strands of endoplasmic reticulum. These structural differences may be the result of microglia being actively engaged in phagocytosis of the large number of degenerating axons which are present in the optic nerve between 8 and 10 weeks post-conception.