The dimerization of avian pancreatic polypeptide (PP) was studied by large-zone gel chromatography using integral boundary analysis. The association constant was determined as a function of temperature and pH. The dimerization is endothermic and entropically driven, which suggests hydrophobic interactions and is enhanced with increasing pH. Analysis of the pH-dependence indicates the involvement of ionizable groups, with pKa values of 4.5-5.5. In the avian PP molecule, there are 6 groups which are potentially titratable in this pH range. A comparison of the amino acid sequence of avian PP with that of the bovine and canine homologs, which also exhibit pH-dependent dimerization, shows that they have 3 carboxylate and 2 guanidinium groups in common. Salt linkages involving these groups may participate in dimerization. In the avian peptide, His-34 may also be involved.