CA 125 has been established as an important tumor marker for monitoring patients diagnosed with nonmucinous ovarian cystadenocarcinoma although it has also been shown to be expressed by other carcinomas, normal epithelial tissues, and fetal tissues. Current evidence implicates a role for CA 125 during early fetal development. The human epithelial amnion WISH cell line is a known secretor of CA 125. WISH cells have been investigated as a model system to characterize the structure of cell-associated and secreted CA 125 of fetal origin. CA 125 secretion was maximal in confluent monolayers of WISH cells where it averaged 2,081 units/ml/24 h. Secretion ranges from 600 to 900 units/106 cells/24 h. [35S]-Methionine-la-belled CA 125 can be detected by 4 h and reached maximal levels of radioactive incorporation in tissue culture medium by 12 h when analyzed by immunoprecipitation with the Ml 1 anti-CA 125 monoclonal antibody and SDS-PAGE, followed by autoradiography. Both cycloheximide and actinomycin D inhibited CA 125 synthesis. CA 125 was demonstrated to incorporate [3H]-galactose but the level of radioactive incorporation was greatly reduced when WISH cells were incubated in the presence of phenyl N-acetyl-α-D-galactosaminide (an inhibitor of O-linked glycosylation) or monensin (an inhibitor of intracellular protein transport within the Golgi complex). Treatment of WISH cells with tunicamycin (an inhibitior of N-linked glycosylation) only slightly decreased label incorporation