The homeodomain transcription factor encoded by the pancreatic and duodenal homeobox gene-1 (Ipf-1) is essential for pancreatic ontogenesis. Whether Ipf-1 is also involved in the neogenesis of beta-cells in the adult pancreas is unknown. We examined whether Ipf-1 is expressed in adult human pancreatic ducts, which are thought to generate new beta-cells. In tissue sections, virtually all duct cells were immunopositive for Ipf-1, as were the islet beta-cells but not the acinar cells. After isolation and culture, both duct and islet cell preparations contained the Ipf-1 immunoreactive proteins p42 and p45 (42 and 45 kDa, respectively) in similar proportions, but the expression levels were twofold lower in duct cells. After 4 h of labeling, the endocrine cells exhibited a sevenfold higher phosphorylation of p42 than the duct cells, whereas p45 was phosphorylated only in endocrine cells. Homeobox binding transcription factor complexes with Ipf-1 in duct cells differed from those in endocrine cells in terms of gel mobility, sequence specificity, and affinity. The observed similarities in Ipf-1 expression by adult human pancreatic duct cells and endocrine cells may reflect their common ontogenic origin, whereas the differences in Ipf-1 phosphorylation and complex formation may correlate with their divergent differentiation.