Vasoactive intestinal peptide (VIP) is a 28-amino-acid peptide belonging to the secretin-glucagon family of peptide hormones. Its sequence is identical in man, ox, pig and rat. VIP is widely distributed in the central and peripheral nervous systems, acting as a neurotransmitter or neuromodulator. Peripherally it is involved in vaso- and bronchodilatation, smooth muscle relaxation and stimulation of secretion. Ectopic endocrine tumors secreting VIP (VIPomas) give rise to raised plasma levels of VIP and are associated with watery diarrhea, hypokalemia and achlorhydria syndrome. VIP shows high sequence homology with peptide histidine-methionine (PHM) in man or peptide histidine-serine (PHI) in pigs or rats, present in the same precursor, and with the neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP). This antibody binds to human, bovine, porcine or rat VIP, but does not bind to PHM or PACAP.