A blocking antibody in pregnant sera seems essential in understanding why a fetus can survive as a semi-allograft in an immunologically competent mother. It has been demonstrated that blocking antibody is closely related to the paternal HLA-DR antigens. However, we previously reported that HLA-DR is not expressed on any trophoblast constituting the fetal frontier at the feto-maternal interface. The present study was then undertaken to clarify whether or not trophoblasts of normal pregnancy and trophoblastic diseases express HLA-DP or HLA-DQ antigens which are closely linked to HLA-DR as a haplotype. Materials were taken from ten pregnant uteri, ranging from 8 to 22 weeks of gestation, two uteri containing complete mole and two uteri each containing gestational choriocarcinoma. Curettage specimens obtained from three cases of complete mole and two cases of partial mole were also employed. Antigen expression was examined by an indirect immunoperoxidase technique using various monoclonal antibodies. From this immunohistochemical study, neither HLA-DP nor HLA-DQ seemed to be expressed on any trophoblast of normal pregnancy and trophoblastic diseases, which, together with the negative findings of HLA-DR on trophoblasts, may suggest that blocking antibody is not generated against HLA-D locus antigens on trophoblasts.