We have developed a new method that permits the complete in vitro construction and selection of peptide or protein libraries. This method relies on an in vitro transcription/translation reaction compartmentalized in water in oil emulsions. In each emulsion compartment, streptavidin (STA)-fused polypeptides are synthesized and attached to the encoding DNA via its biotin label. The resulting protein-DNA fusion molecules recovered from the emulsion can be subjected to affinity selection based on the properties of the peptide portion, whose sequence can be determined from that of its DNA-tag. This method, named 'STABLE' (STA-biotin linkage in emulsions), should be useful for rapid in vitro evolution of proteins and for ligand-based selection of cDNA libraries.