By transmission electron microscopy we studied 38 epiretinal membranes obtained from human eyes by vitrectomy operation in rhegmatogenous retinal detachment. Two of the membranes were composed of closely packed glial cell processes. The cytoplasm of the glial cell processes in both membranes contained little besides loosely packed filaments and microtubules. Microvilli protruded from the cells at the vitreal surface of one of the membranes. Junctional complexes were present between the cells only at the vitreal surface. In the other glial membrane, microvilli extended into a duct-like structure with junctional complexes sealing the lumen. A discontinuous basal lamina was laid down by cell processes at the retinal side of the membranes. Similar glial membranes, formed by migration and proliferation of astrocytes from the retina, were seen in owl monkeys with experimental and naturally occurring retinal detachment. A new epiretinal membrane (reformed in one of the eyes from which a dense glial membrane had been removed three weeks previously), was composed primarily of single large cells with extensively developed rough endoplasmic reticulum. The new cells were surrounded by collagen, fine fibrillar material, and fibrin. The new membrane also contained free pigment granules and pigment-containing macrophages. Additionally, we observed spicule-like crystals between the cells in the glial membrane specimens, identified as calcium phosphates by energy-dispersive x-ray analysis.