Fibronectins, high molecular multifunctional glycoproteins of the extracellular matrix and plasma, have been a popular area of research in the pathogenesis of proliferative disorders of the retina. Several immunohistochemical studies have revealed that fibronectin is a major constituent of epiretinal membranes and that the cell types involved in proliferative intraocular disorders may synthesise it. However, owing to the fact that plasma and cellular fibronectin are similar in their overall structure, the origin of fibronectin in epiretinal membranes has not yet been clearly defined. In this study, we used two monoclonal antibodies: FN-3, which recognises an extra domain present in the cellular but not plasma form of fibronectin; and FN-4, which reacts with an antigenic site on both plasma and cellular fibronectin. In 37 epiretinal membranes obtained from eyes with proliferative vitreoretinopathy and proliferative diabetic retinopathy, we demonstrated the presence of cellular fibronectin, thus indicating local production. The significantly stronger and positive immunostain with FN-4 in the same specimens suggests the colocalisation of plasma fibronectin, that may be derived from the breakdown of the blood-retinal barrier and trapped in membranes during their formation. In pathological vitreous we demonstrated both types of fibronectin by western blot analysis.