A phage display system was developed and used to select zinc finger proteins with altered DNA-binding specificities. The three zinc fingers of the Zif268 protein were expressed on the surface of filamentous phage, and a library of variants was prepared by randomizing critical amino acids in the first zinc finger. Affinity selections, using DNA sites with base changes in the region recognized by the first finger, yielded Zif268 variants that bound tightly and specifically to the new sites. This phage system provides a tool for the study of protein-DNA interactions and may offer a general method for selecting zinc finger proteins that recognize desired target sites on double-stranded DNA.