Expression of HLA class I molecules in trophoblast cells from various locations in normal human first trimester and term placenta was investigated by immunohistochemistry with a panel of monoclonal antibodies against the heavy chains or complete HLA class I molecules complexed with beta2-microglobulin. These reagents were also employed to distinguish between the products of different HLA class I loci. In addition to previously characterized reagents, a novel monoclonal antibody against HLA-A molecules (TU155) was used. Various choriocarcinoma and transfected cell lines served as controls for the specificities of the monoclonal antibodies. Cells in close contact with maternal cells, such as invading trophoblast cells and cells of the basal plate, expressed beta2-m micro globulin in association with HLA-G and HLA-C heavy chains. These class I heavy chains may also have been present as isolated molecules, although not in each of the cells. In contrast, cells of the chorion laeve exclusively expressed HLA-G, and not HLA-A, -B, or -C antigens. Our data support the often discussed immune protective function and the regulatory function of the HLA-G molecule, during invasion. In addition, by using monoclonal antibodies HCA2 (anti-HLA-A and -G), HC10 (anti-HLA-B and -C), TU149 (anti-HLA-B, -C, and some -A alleles), SFR8-B6 (anti-HLA-Bw6 and some -C), LA45 (some HLA-A and -B), TU48 (anti-HLA-Bw4 and some -A), and TU155 (anti-HLA-A), we show the presence of HLA-C molecules in all extravillous trophoblast cells of the cell columns and in the basal plate; the trophoblast cells of the chorion laeve lack this antigen. The function of this molecule is not clear, although a protective function against natural killer cell activity in the endometrium is postulated.