Ideal gene therapy vectors would be delivered intravenously to transfect only specific cells. Existing vectors only transfect cells in vivo in a manner determined by blood flow and the site of introduction. As a general and systematic approach for generating cell-targeting ligands for gene therapy vectors, we have used peptide-presenting phage libraries to select peptides that bind and enter several different cell types. Because of their small size, cell-binding peptides such as these could be incorporated into biological or physical gene therapy vectors. In addition, peptide-presenting phage themselves may also be candidates for gene therapy vectors.