We developed a mouse monoclonal antibody, S2n8, by immunizing mice i.p. with human decidual cells collected in the first trimester of pregnancy. By indirect immunofluorescence staining of frozen sections, S2n8 was found to react with decidual cells and endometrial stromal cells throughout the menstrual cycle, but not with endometrial glandular cells or with the endometrial surface epithelium. Judging from the fluorescence intensity, the antigen expression on stromal cells was weak in the proliferative phase, and became stronger in the secretory phase. Decidual cells in the first trimester of pregnancy and decidual cells at term showed strong expression of this antigen. Indirect immunofluorescence staining of enzymatically dispersed decidual tissue revealed that the S2n8 antigen was expressed on the decidual cell surface. Flow cytometric analysis of 12 freshly prepared stromal cell-enriched cell suspensions showed that 74.8-94.2% (mean +/- SD 86.1 +/- 6.6%) of the cells carried the antigen. The expression of S2n8 antigen on cultured stromal cells was enhanced by the addition of oestradiol and/or progesterone. The antigenic molecule was purified by immunoaffinity chromatography from decidua collected in the first trimester of pregnancy, and the molecular weight was estimated to be approximately 140 kDa. These findings indicate that the S2n8 antigen is a useful cell surface marker for stromal cells/decidual cells and is associated with their differentiation.