CD30 (Ki-1) antigen has been considered to be expressed on hematopoietic cells including the ones of the recently described anaplastic large cell lymphoma (ALCL), the Reed-Sternberg (RS) cells of Hodgkin's disease and the scattered large parafollicular cells in normal lymphoid tissues. Since then, several reports have been published describing CD30 expression in non-hematopoietic and malignant cells, such as cultivated human macrophages, human decidual cells, histiocytic neoplastic cells, mesothelioma cells, embryonal carcinoma, and seminoma cells. In the present study, we investigated the immunohistochemical expression of CD30 antigen in 15 paraffin-embedded placentas from fetuses after spontaneous abortion in the first trimester of gestation (8th, 10th, and 12th week, respectively) using the monoclonal antibody Ber-H2. All the pregnant patients had been given hormonal medication to support gestation. In addition, a panel of monoclonal antibodies for the identification of leukocytes (CD45/LCA), B-lymphocytes (CD20/L-26), and T-lymphocytes (CD45RO/UCHLI) was performed. Our findings were correlated with those found in 15 placentas obtained from 15 fetuses at the same time, after therapeutic or voluntary abortions. This study demonstrates that, 1) decidual endometrial stromal cells are able to express the CD30 (Ki-1) antigen, 2) the expression of CD30 in decidual cells is higher in cases of hormonal administration (to support gestation), than that found in normal gestation. In the former cases (hormonal support of gestation), a mild mononuclear infiltration of the decidua by UCHLI (T marker) positive cells, accompanies the CD30 positive cells.