Monocyte/macrophages play a prominent role in several forms of retinal pathology including proliferative vitreoretinopathy, senile macular degeneration, and retinal wound healing. In each of these entities, the retinal pigment epithelium (RPE) is characteristically involved as well. Since RPE cells are known to secrete chemoattractants for astrocytes, we considered the possibility that they might secrete chemotactic factors for monocytes in addition. We have found in in vitro assays that a 5% concentration of medium from 6 different well-established RPE culture lines each consistently induced monocyte migration greater than that elicited by either buffer or unconditioned medium. "Checkerboard" analysis indicated that RPE culture supernatants induced optimal migration with a stimulus gradient (chemotaxis as opposed to chemokinesis alone). Chemotactic activity could be detected in eluates from ion exchange high performance liquid chromatography or gel filtration columns. Several peaks of activity suggested that more than one factor may be responsible for the ability to induce cell migration. The chemotactic activity was largely heat stable. The chemotactic factor induced only minimal migration of polymorphonuclear leukocytes. The secretion of chemotactic factors for monocytes could contribute significantly to the pathogenesis of several retinal diseases.