The endometrial stroma is transformed into decidua by an adequate hormonal stimulus or by pregnancy, the endometrial stroma is transformed into decidua. There is increasing evidence, that the decidua has a central functional relevance for the survival of the mammalian embryo in an immunological competent host. In this study, the decidua of 10 patients were examined after therapeutic abortion in the 8th-10th week of pregnancy. Various monoclonal antibodies were used for immunophenotyping of lymphoreticular cells. Most intradecidual lymphoreticular cells could be identified as Ki-M6+ and Ki-M8+ macrophages, which also expressed HLA-DR (class-II) antigens. This finding is underlined by the fact, that Tü-35+ cells occurred in comparable numbers as macrophages and B-lymphocytes were virtually absent from the decidua. Next in frequency were OKT3-positive T-lymphocytes. Most of the lymphoid cells were detected by monoclonal antibody OKT8 (suppressor/cytotoxic lymphocytes) while OKT4+ helper/inducer cells occurred only in low numbers. Though definite conclusions in the cells' functional properties cannot be drawn from an immunohistological study, our findings might be regarded as a basis to explain the tolerance of the embryo in this apparently privileged immunological site.