Phage display-based method to search for cell-internalizing mAbs. The phage antibody library was “enriched” by affinity panning against the desired antigens. Plasmids encoding scFvs were collected from TG1 E coli strains infected by “enriched” phage libraries. Genes of scFvs were digested out and ligated into a PSIF fusion protein expression vector. These plasmids were then transformed to TG1, and then single colonies were picked up. From these individual colonies, monoclonal scFv-PSIF fusions were induced in TG1 supernatants. Using these fusion proteins, binding affinities and internalizing activities of several hundreds of scFv-PSIFs were easily estimated by ELISA and cytotoxicity assays, respectively. Finally, genes of positive scFvs were collected from TG1, and cell-internalizing scFvs were identified by sequencing. In this report, we used anti-Robo4 and anti-VEGFR2 immune phage scFv libraries as the phage antibody libraries, and mRobo4 and mVEGFR2 as the desired antigens.