The capacities of porcine choroidal fibroblasts, retinal glial cells, and retinal pigment epithelial cells to contract collagen gels in vitro were compared. Experiments with varied cell numbers indicated that glial cells are the most effective, followed by choroidal fibroblasts and retinal pigment epithelial cells. Analysis of the secretory products from cultures of these cell types revealed that retinal pigment epithelial cells synthesize and secrete peptides that promote fibroblast contraction of collagen gels in vitro. The mechanism of action of the retinal pigment epithelial cell- secreted contraction promoter was compared with that found in serum (type A) and secreted by cultured endothelial cells (type B). Like the serum factor, the retinal pigment epithelial cell-secreted factor was not dependent on active protein synthesis by the target cell and must be present continuously to promote contraction.