Five morphologically distinguishable cell types were observed in 56 epiretinal and vitreous membranes obtained surgically from eyes with various ocular diseases: (1) retinal pigment epithelial (RPE) cells that were evident only in association with retinal detachment, (2)(macrophages, (3) fibrocytes, (4) fibrous astrocytes that were characteristic of all disease groups, and (5) myofibroblast-like cells that had mostly the characteristics of fibrocytes and, occasionally, of RPE cells or fibrous astrocytes. The combination of cell types varied in different types of epiretinal membranes, but the formation of collagen and the development of cells with myofibroblast-like properties were common features and seemed to be within the capacity of several cell types. These two common features seem to be the basis for the contractile properties of epiretinal and vitreous membranes.