The contractile activity of cells in epiretinal membranes leads to retinal detachment and proliferative vitreoretinopathy, but it is unknown which cell type contributes most to this process. In this study, the relative contractility of three cell types thought to be involved in epiretinal membrane formation was measured. Bovine scleral fibroblasts (BSF), retinal pigment epithelium (BRPE) and retinal glia (BRG) were suspended in a rapidly polymerizing floating type I collagen matrix. Matrix contraction was assessed and morphologic studies were performed using light microscopy and scanning electron microscopy. BSF reached 50% contraction within 4 days and BRPE needed 7-10 days, whereas BRG showed no matrix contraction. Cell proliferation, as shown by bromodeoxyuridine labeling, did not occur in the matrix. During the period of greatest contraction, cells had formed processes but were isolated from each other. Over the 7 day period, an increasing number of cells was found on the surface of the matrix, resulting (with BSF) in an encapsulating monolayer. This monolayer developed only after most of the contraction was over. These findings suggest that BSF and BRPE reorganize the collagen matrix through a combination of individual cellular mobility and attachment to the collagen fibers. BRG's inability to attach to the collagen and reorganize its arrangement resulted in its lack of matrix contraction.