A panel of monoclonal antibodies directed at fucosylated and sialylated carbohydrates on the Type 1 and Type 2 blood group precursor chains was used in an immunohistological study of trophoblast subpopulations. Formalin-fixed paraffin-embedded sections of normal trophoblast throughout pregnancy, hydatidiform moles and gestational choriocarcinoma were examined. Sialyl-Lex was localized in extravillous trophoblast populations in normal and molar pregnancies and choriocarcinoma tumour cells. After neuraminidase treatment, Lex was identified in similar cell populations. Although Lea was identified in syncytial trophoblast cells at the maternofetal interface, no staining was seen for sialyl-Lea. Villous trophoblast did not stain with any of the monoclonal antibodies used. Our results underline the divergent differentiation of trophoblast into villous and invasive extravillous subpopulations. The analogy with tumour cells is discussed.