In vitro antibody-display technologies are powerful approaches for isolating monoclonal antibodies from recombinant antibody libraries. However, these display techniques require several rounds of affinity selection which is time-consuming. Here, we combined mRNA display with a microfluidic system for in vitro selection and evolution of antibodies and achieved ultrahigh enrichment efficiency of 10(6)- to 10(8)-fold per round. After only one or two rounds of selection, antibodies with high affinity and specificity were obtained from naive and randomized single-chain Fv libraries of approximately 10(12) molecules. Furthermore, we confirmed that not only protein-protein (antigen-antibody) interactions, but also protein-DNA and protein-drug interactions were selected with ultrahigh efficiencies. This method will facilitate high-throughput preparation of antibodies and identification of protein interactions in proteomic and therapeutic fields.