Human leucocyte antigen (HLA)-C is expressed at lower levels than other classical HLA-I molecules on somatic cells. Surface HLA-C proteins can occur as conventionally β₂-microglobulin (β2m)-associated complexes or as open conformers dissociated from peptide and/or β₂m. We investigated the conformation of HLA-C molecules on normal human trophoblast cells, which invade the maternal decidua during placentation. A panel of monoclonal antibodies to different conformations of HLA-I molecules was used in flow cytometry and surface immunoprecipitation experiments. On the surface of trophoblast cells only β₂m-associated complexes of HLA-C molecules were detected. In contrast, both open conformers and β₂m-associated HLA-C could be detected on other cells from the decidua, HLA-C-transfectants and cell lines. The levels of HLA-C expressed on primary trophoblast cells could be detected by antibodies specific to non-β₂m-associated conformations because binding was seen after acid-induced denaturation of surface proteins. In contrast to HLA-G molecules on trophoblasts, we found no evidence for the presence of disulphide-linked multimers of HLA-C complexes. These results show that most HLA-C molecules present at the trophoblast cell surface are in the conventional β₂m-associated conformation. These findings have implications regarding the stability of trophoblast HLA-C molecules and how they interact with receptors on decidual leucocytes during placentation.