a novel helper phage that improves phage display selection efficiency by preventing the amplification of phages without recombinant protein phage display is a widely used technology for the isolation of peptides and proteins with specific binding properties from large libraries of these molecules a drawback of the common phagemidhelper phage systems is the high infective background of phages that do not display the protein of interest but are propagated due to nonspecific binding to selection targets this and the enhanced growth rates of bacteria harboring aberrant phagemids not expressing recombinant proteins leads to a serious decrease in selection efficiency here we describe a vcsm13derived helper phage that circumvents this problem because it lacks the genetic information for the infectivity domains of phage coat protein piii rescue of a library with this novel ct helper phage yields phages that are only infectious when they contain a phagemidencoded piiifusion protein since phages without a displayed protein carry truncated piii only and are lost upon reinfection importantly the ct helper phage can be produced in quantities similar to the vcsm13 helper phage the superiority of ct over vcsm13 during selection was demonstrated by a higher percentage of positive clones isolated from an antibody library after two selection rounds on a complex cellular target we conclude that the ct helper phage considerably improves the efficiency of selections using phagemidbased protein libraries