Human chorionic gonadotropin (hCG) is a placental hormone and marker for the differentiation process of cytotrophoblast cells to syncytial trophoblasts. It is secreted into the maternal circulation and urine. The glycoprotein hCG is also found in the serum and urine of patients with malignant trophoblastic diseases. For this study hCG was purified from pooled human mid-trimester amniotic fluid, serum and urine of pregnant women and from supernatants of trophoblast tumour cell cultures Jeg3 and BeWo by immunoadsorption chromatography with specific hCG-antibodies. The purity and identity of the isolated hCG were checked by Western blot analysis. Sialylated oligosaccharides of the Lewis type of isolated hCGs were analysed by dot blot and ELISA technique with monoclonal antibodies specific for sialyl Lewis x and sialyl Lewis a-carbohydrate epitopes. BeWo and Jeg 3 cells were additionally investigated by immunofluorescence. HCG isolated from the serum and amnion of pregnant women bear both sialyl Lewis x (sLex)-antigen and sialyl Lewis a (sLea)-antigen. The same result was observed for hCG isolated from the supernatants of the trophoblast tumour cell line Jeg3. On the contrary, hCG isolated from the supernatants of the trophoblast tumour cell line BeWo showed only a very weak expression of these antigens. HCG isolated from the urine of pregnant women was negative for both sialyl Lewis antigens. The results are discussed with respect to a possible relationship to selectin- mediated cell adhesion processes. This could play a role in the prevention of leukocyte adhesion on the foetal syncytiotrophoblast. Glycosylation of hCG in trophoblast tumour cells, on the other hand, could increase the metastatic activity of these cells.