The capacity of membrane glycoproteins to interact with proteinases was investigated in the model system: Membrane sialoglycoprotein from human erythrocytes (glycophorin) and lysosomal proteinases from rat liver. Glycophorin was found to stimulate the activity of a lysosomal proteinase mixture up to about 150% at pH 6.9. Cathepsin L was found to be the primarily stimulated proteinase. The stoichiometry in the saturation range of the dose-response curve waas about 10 to 20 molecules glycophorin per molecule cathepsin L. The mechanism of the activation is unknown. Interactions of this type may be of importance for the regulation of cell proliferation on the level of cell membranes.