Though the pathogenesis of choroidal neovascular membranes is uncertain, there is evidence to support a primary dysfunction in the retinal pigment epithelium (RPE). This suggests the possibility that a healthy RPE may provide a physical and/or chemical barrier to subretinal endothelial cell invasion. It has recently been shown that RPE cells in culture produce an inhibitor of neovascularization. Histopathologic evidence suggests that RPE cells tend to surround new blood vessels and contain them. We therefore investigated the possibility that RPE cells are guided toward endothelial cells by chemoattractants. Using a modified Boyden chamber technique, we showed that endothelial cells in culture produce a chemoattractant for RPE cells. The active component is trypsin sensitive, stable at extremes of pH (3 through 10), and nondialyzable (12,000- to 14,000-dalton cutoff). It is partially heat stable but becomes completely heat stable in the presence of 1% sodium dodecyl sulfate. These are all characteristics of the previously described endothelial cell-derived growth factor, suggesting that this mitogen might be the chemoattractant. The ability of RPE cells to be attracted to sites of new blood vessel formation may enhance their potential function as inhibitors of neovascularization.