Proteases have been used as a tool to investigate the role of cell-surface molecules of cultured retinal pigment epithelial cells (RPE) in the phagocytosis of rod outer segments (ROS). Proteolytic digestion of RPE cells by pronase, thermolysin and Staphylococcus aureus V8 protease (V8 protease) inhibited the phagocytosis of ROS without affecting the viability of the RPE cells. A particular feature of RPE cell proteolysis was that those macromolecules responsible for ROS ingestion were susceptible, while those macromolecules that mediated ROS binding were resistant to cleavage by all three proteases. By taking advantage of this phenomenon, ROS were used as affinity particles to obtain a plasma membrane-enriched fraction of RPE cells before and after proteolytic digestion. All three proteases partially or completely removed several glycoproteins from the cell surfaces. Removal of these glycoproteins was correlated with a loss in phagocytic ability by RPE cells. Two high-molecular-weight (MW) glycoproteins of MWs 160 000 and 214 000 were consistently removed by all proteases tested. Protease-treated RPE cells restored their phagocytic capabilities and normal glycoprotein composition within 24 hr after proteolytic treatment. These data suggest that glycoproteins located on the surfaces of RPE cells may be involved in mediating the phagocytosis of ROS by these cells.