• In 14 patients, preretinal membranes, causing retinal traction and severe visual impairment, were removed by vitrectomy and evaluated by light and electron microscopy using myosin subfragment-1 to stain actin filaments. Eight membranes were of vascular origin, six of nonvascular origin. All but one contained bundles of oriented actin filaments within a number of their nonvascular stroma cells, suggesting that the contractile protein action may have been involved in their clinically observed contraction.