It has been shown that sequence-specific DNA-binding domains containing zinc fingers can be selected from libraries displayed on filamentous bacteriophage. The affinity and specificity of these peptides are well characterised in vitro, but few data are available to demonstrate specific DNA binding and discrimination between closely related DNA sequences in vivo. Transient transactivation assays were performed in mammalian cells, using expression plasmids which produce different amounts of a model transcription factor containing a phage-selected zinc finger DNA-binding domain, and reporter plasmids which carry systematic variations of the promoter sequence. When the intracellular concentration of the transcription factor was appropriate, activation of gene expression was absolutely dependent on a promoter having the same DNA sequence as that originally used to select the zinc finger domain by phage display. However, excessive intracellular concentrations of the transcription factor resulted in some less-specific DNA binding, leading to gene activation from similar promoters containing a maximum of two base changes. Thus, provided delivery is carefully controlled, highly specific control of gene expression in vivo can be achieved using artificial transcription factors containing phage-selected zinc finger DNA-binding domains.