Mistletoe lectins (MLs) are the active components of aqueous mistletoe extracts widely used in complementary cancer therapy, however, it is not clear if they bind to carbohydrate residues only or whether they interact with proteins as well. Protein-protein interactions do not seem unlikely as MLs act at very low molar concentrations usually observed with peptide-peptide interactions only and not seen with lectin-sugar interactions.In order to detect protein-protein interactions a random peptide library was screened for the ability to bind to MLs.MLs bound to peptides showing homologies to multidrug resistance-associated protein 5 (MRP5). However, the MLs only slightly modified the MRP5 efflux pump, while periodate treatment to inhibit cell membrane binding via glycan completely abolished the ML-I binding sites in MRP5 overexpressing cells.The protein sequence is not important for ML-I binding, indicating that the biological activity of MLs can most likely be attributed to the sugar chains.