The present invention relates to polypeptides containing an antigen-binding domain and a carrying moiety having an inhibiting domain that inhibits the antigen-binding activity of the antigen-binding domain, and having a longer half-life than that of the antigen-binding domain existing alone； methods for producing and screening for the polypeptides； pharmaceutical compositions containing the polypeptide； methods for producing and screening for a single-domain antibody whose antigen-binding activity is inhibited by its association with particular VL, VH or VHH； and fusion polypeptide libraries including a single-domain antibody whose antigen-binding activity is inhibited by its association with particular VL, VH or VHH
