The present invention provides novel anti-CD26 antibodies and other, related polypeptides, as well as novel polynucleotides encoding the antibodies and polypeptides. The invention also provides methods of making the antibodies and polypeptides. Compositions and cells comprising the antibodies or polypeptides are further provided. Methods of using the antibodies and/or polypeptides, such as to inhibit cell proliferation and in the treatment of conditions associated with CD26, are also provided.