Antiestrogen, such as tamoxifen, selectively increased the production of secreted protein of 37,000 molecular weight (Mr) in estrogen receptor positive human breast cancer MCF-7 cells but not in estrogen receptor negative MDA-MB-231 cells, and the production of this protein by antiestrogen was inhibited by concomitant estradiol treatment. Proteins were detected by [$^{35}S$]methionine and [$^{35}S$]cysteine labeling of the cells and analysis of proteins by sodium dodecyl sulfate-polyacrylamide gels and autoradiography. Enhanced production of 37,000 Mr protein was observed within 6 h of antiestrogen treatment, with maximal synthesis seen at 1~2 days when this protein represents about 6% of the total radiolabeled secreted proteins. This protein was stimulated maximally by $10^{-8}$ M trans-hydroxytamoxifen or LY 117018 or $10^{-6}$ M tamoxifen, and its antiestrogen specificity was seen by the fact that transtamoxifen increased this protein, whereas cis-tamoxifen, an estrogen, does not. Interestingly, the basal level of synthesis of the 37,000 Mr protein was high in the absence of estrogen and was then stimulated only minimally by the addition of antiestrogen, suggesting that this protein was dearly produced as an estrogen antagonistic protein. Amino acid incorporation conducted in the presence of tunicamycin and endoglycosidase H indicated that this protein was glycoprotein. This protein might serve as an useful marker for anti estrogen action in MCF-7 human breast cancer cells.