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CEACAM1 (biliary glycoprotein or CD66a) is a member of the carcinoembryonic antigen (CEA) subgroup of the CEA family. Eleven RNA isoforms derived from the splicing of a single CEACAM1 gene have been described. Some of the CEACAM1 isoforms have been recognized by the CD66 antibodies in T and B lymphocytes, natural killer cells, granulocytes and epithelial cells in several human tissues. Although it is also present in soluble form in bile and serum, and elevated levels have been found in the serum of patients with liver diseases, it is not known which isoforms are primarily involved. In order to learn more about the distribution and properties of particular CEACAM1 isoforms, we have prepared a monoclonal antibody specific for the A2 domain of CEACAM1, designated TEC-11. This antibody does not cross-react with other members of the CEA family. Immunoblotting analysis revealed that the TEC-11 epitope was present in all cell types expressing CEACAM1 containing the A2 domain [CEACAM1(A2)], including granulocytes (160 000 MW isoform) and sperm cells (140 000 MW isoform). A 115 000 MW isoform of CEACAM1(A2) was present in human serum, bile, saliva and seminal fluid. Human bile, saliva and seminal fluid also contained the 160 000 MW CEACAM1(A2) isoform. Significantly higher serum levels of the 115 000 MW CEACAM1(A2) isoform were detected in patients with obstructive jaundice. The 160 000 MW isoform of CEACAM1(A2) in bile, but not a 115 000 MW isoform in serum and bile, carried the 3-fucosyl-N-acetyl-lactosamine moiety. The combined data indicate that various isoforms of CEACAM1(A2) are present in different body fluids where they could take part in different CEACAM1-mediated functions.