Human neuropeptide Y (hNPY) is a potent vasoconstrictor peptide of 36 aminoacid residues. We isolated hybridomas secreting four monoclonal antibodies directed against various epitopes of neuropeptide Y and studied their cross-reactivity with peptide YY (PYY) and the pancreatic polypeptide (PP), two peptides sharing sequence homologies with hNPY (respectively 70% and 50%). The antibody NPY02 is an IgG1 with a Ka of 5.5 x 10(10) liters/mol. It binds to the 11-24 region of NPY (IC50 = 2 x 10(-7)M), does not recognize PP but cross-reacts weakly with PYY. Antibodies NPY03 and NPY05 are IgG2 with respective Ka's of 6.7 x 10(9) and 2.5 x 10(10) liters/mol. They interact with a C-terminal epitope on NPY (NPY 27-34, IC50 = 2 x 10(-9) M for NPY03 and NPY 32-36, IC50 = 1 x 10(-9) M for NPY05). These two antibodies cross-react with PYY whereas only NPY05 binds PP. NPY05 is unable to bind the free acid form of neuropeptide Y. The 32-36 COOH free subpeptide is recognized 50,000 less efficiently by NPY05 than its amidated form. Antibody NPY04 is an IgG3 with a Ka of 3.8 x 10(8) liters/mol. It recognizes a N-terminal epitope between aminoacids 1 and 12 (IC50 = 2.5 x 10(-6) M). NPY04 interacts weakly with PYY but not detectably with PP. These results obtained with 4 different monoclonal antibodies demonstrate the presence of at least four epitopes on hNPY, two of them being continuous. These antibodies will be used to study the interaction of NPY with its receptor and to develop sensitive and specific assays for determination of NPY concentrations in biological fluids.