Tissue localization of a human pancreas cancer-associated antigen (PCAA) and carcinoembryonic antigen (CEA) was studied in a homologous pancreatic tumoral xenograft, a human pancreatic cancer line established from ascites (AsPC-1), with the use of the indirect immunofluorescence technique with specific anti-PCAA and anti-CEA antisera. Histologically, AsPC-1 xenograft was composed of mucinous adenocarcinoma of variable size and degree of glandular differentiation. PCAA was selectively associated with the columnar cytoplasm, was involved primarily in the epithelial proliferation, and originated at the basal aspect of the glands. In contrast, CEA appeared diffuse and was found predominantly in the cuboidal cells, with higher intensity at the luminal border and in mucin. Furthermore, growth comparison of the AsPC-1 adenocarcinoma revealed that, except in the juvenile gland that was morphologically consistent with grade 1 (G1) carcinoma in situ, the number of PCAA-positively stained cells decreased from G1 to grade 4 epithelial differentiation. In well-differentiated adenocarcinoma and degenerating adenocarcinoma, cells positive for PCAA were released into the lumen of the glands, whereas in poorly differentiated adenocarcinoma these cells were often scattered singly and/or in focal clusters and disappeared in very poorly differentiated adenocarcinoma. This study showed that the PCAA was different from CEA in its immunologic reactivity and distribution in its homologous tumoral tissues, in which PCAA was predominantly associated with the proliferative phase of the malignant epithelium. These results also indicated the potential applicability of this antigenic expression in the etiology, disease grading, and staging of human pancreatic cancer.