Phage antibody technology is a powerful approach for generating human antibodies to virtually any target antigen. For many therapeutic applications, it is useful to generate antibodies that bind to cell-surface receptors in a manner where binding results in internalization of the antibody. This allows use of the antibody to deliver toxic payloads intracellularly to achieve a therapeutic effect. Here we describe how phage antibody libraries can be directly selected on tumor cell lines to generate antibodies binding cell-surface receptors and which are rapidly internalized upon binding. Protocols are provided showing how to (1) directly select internalizing antibodies from phage antibody libraries; (2) screen phage antibodies in a high-throughput flow cytometry assay for binding to the tumor cell line used for selection; (3) identify the antigen bound by the phage antibody using immunoprecipitation and mass spectrometry; and (4) verify and quantitate such that phage antibodies are internalized.