Glycodelin is a 28 kDa glycoprotein of the lipocalin family that was previously considered to be specific for the reproductive tract. Glycodelin is found in the secretory glandular epithelium of endometrium and in seminal vesicles. Given the cyclic differentiation of normal endometrial epithelium, we studied by immunohistochemistry a possible expression of glycodelin in other tissues displaying stroma-to-epithelium maturation. We report here that 11/11 biphasic synovial sarcomas expressed glycodelin in the cells exhibiting epithelial or glandular differentiation while the sarcomatous spindle cells remained negative. Glycodelin was also found in secreted material in the lumina of the gland-like structures. In only 1 of the 7 monophasic synovial sarcomas studied, focal glycodelin reactivity was seen in some flattened spindle cells. The expression of glycodelin in biphasic synovial sarcoma tissue was further verified by the demonstration of glycodelin mRNA by reverse transcriptase-polymerase chain reaction. Considering the monoclonal origin of synovial sarcomas, our findings raise the intriguing possibility that activation of expression of the glycodelin gene is involved in the molecular regulation of mesenchyme-to-epithelium differentiation.