The thymus gland produces several hormones or hormone-like substances which are derived from a polypeptide precursor containing (in the rat) 113 amino acids and known as prothymosin alpha. A peptide containing 28 amino acid residues, named thymosin alpha 1, was originally isolated from calf thymosin fraction 5 and shown to restore various aspects of immune function in several in vitro and in vivo test systems. Thymosin alpha 1 was subsequently isolated from a similar fraction from human thymosin and reported to have the same amino acid sequence as bovine thymosin alpha 1.