Quantitative, autoradiographic, or immunohistochemical light microscopy was conducted on 85 surgically excised epiretinal membranes to investigate the activities of component cells and the natural history of the extraretinal scars. Membranes of less than four months' clinical duration ('early' membranes) contained significantly more cells than 'late' membranes, while collagen was more abundant in the older specimens. No correlation was established between membrane opacification and either the cellularity or the collagen content of the membranes. Epiretinal membranes had some features in common with healing skin wounds, but the activities of cells in the epiretinal membranes were relatively protracted and disordered. Fibronectin was found to be a significant component of epiretinal membranes and therefore represents a target at which pharmacological intervention could be aimed.