The immunoreactivity of monoclonal antibody (MoAb) B72.3 with ovarian serous tumors of borderline malignancy from 44 women who were pregnant, were on hormone medication containing a progestin, or were known to be in the secretory phase of the menstrual cycle, was compared with that of similar tumors of 32 patients who were not known to be in any of these three categories. All 76 borderline tumors expressed the tumor-associated glycoprotein (TAG-72) recognized by MoAb B72.3. Striking staining differences (P less than 0.0001) were observed between the hormone-related and the nonhormone-related tumors. Differences were also noticed between the staining of tumors from pregnant patients and that of previous, persistent, or recurrent tumors of the ipsilateral or contralateral ovaries when the same patients were not pregnant. Tumor MoAb B72.3 reactivity increased with progressive gestational age and fell to lower levels at term and during the postpartum period. Although it has been suggested by cell culture studies, enhanced TAG-72 expression in human tumors under hormonal stimulation has not been described before.