Although decidual stromal cells (DSC) have classically been considered to play a nutritional role during pregnancy, several reports have demonstrated that they can also exert different immune activities. Furthermore, some authors have occasionally found antigens on DSC normally expressed by immune cells. In this study, we isolated and cultured 12 human DSC lines and studied them with immunocytochemistry and flow cytometry using monoclonal antibodies against antigens associated with hematopoietic cells. Decidual stromal cells exhibited a constant phenotype: they were CALLA (CD10)-positive and DR-positive, although the expression of CD45, the leukocyte common antigen, was found to be very weak or negative. We also detected myelomonocytic antigens CD11b (CR3), CD13, CD16 (Fc gamma RIII) and CD36, although DSC lacked CD14, CD15 and CD33. B cell antigens CD20, CD21 (CR3), CD23 (Fc epsilon RII) and CD24 were expressed. DRC-1, an antigen detected on follicular dendritic cells (FDC), was also observed on DSC. When these cells were cultured in the presence of progesterone, they expressed desmin and prolactin (PRL), findings that confirmed their identity as DSC. The phenotype described, together with the immune activities reportedly carried out by DSC, suggest that DSC may play a role in the maternal-fetal immune relationship.