Co-crystallization of membrane proteins with antibody fragments may emerge as a general tool to facilitate crystal growth and improve crystal quality. The bound antibody fragment enlarges the hydrophilic part of the mostly hydrophobic membrane protein, thereby increasing the interaction area for possible protein-protein contacts in the crystal. To serve as a ‘crystallizing ligand’, the antibody fragment has to recognize an epitope that is only present in the native conformation (and not in the denatured state) of the membrane protein, bind with high affinity, and form stable and rigid complexes. In an effort to develop a fast and reliable method for generating crystallizing ligands, we are constructing immune Fab libraries against various mammalian GPCRs and transporters in a phage display format. Protocols for immobilization of detergent-solubilized membrane proteins during selection, for ELISA screening, and for BIAcore evaluation will also be discussed.