Thirty human endometria were selected from women aged 21-54 years who had undergone routine dilation and curettage procedures for tubal ligation, infertility dating, and irregular menstrual cycling. Histologic sections of the cases chosen were examined to exclude any major pathologic condition (including chronic endometritis). The specimens were stained with monoclonal antibodies to a common leukocytic antigen (H Leu-1 and PD7/26), pan-T-cell antigen (UCHT1), T helper/inducer and T suppressor/cytotoxic antigens (Leu-3a and UCHT4, respectively), pan-B cell antigen (To15 and Leu-12), and macrophage antigens (UCHM1 and Leu-M3). Other antibodies used included TAL-1B5 (anti-HLA-DR), Leu-7 (natural killer cell) and Na 1/34 (anti-T6/Langerhans/interdigitating reticulum cell). The endometria contained significant numbers of common leukocyte antigen-positive cells (occupying approximately 10-15% of the stroma), the numbers of which appeared to increase in the late secretory/pre-menstrual phase (20-25% of the stroma). The major leukocyte populations were T cells and macrophages; the latter, with neutrophils, appeared to account for the premenstrual increase in leukocytes. T cells were distributed both diffusely in the stroma and in periglandular stromal aggregates closely applied to the glands. The T8+ suppressor/cytotoxic population was predominant within the stromal nodules. In addition, scattered intraepithelial T suppressor/cytotoxic cells were present. Macrophages (UCHM1 and HLA-DR+) were also distributed diffusely in the stroma and as part of the periglandular stromal aggregates, in areas sending long cell processes into the epithelium. B cells appeared to be limited to scattered cells in the stroma, only increasing in number within lymphoid follicles. Natural killer cells, as defined by Leu-7+ cells, were also present, scattered singly in the stroma and within lymphoid follicles. The demonstration of large mononuclear dendritic-appearing Na 1/34+ cells within the glands of the endometrium in 5/30 cases suggests the presence of T6+ Langerhans/interdigitating reticulum cells in the endometrium. Thus, the normal endometrium has an important population of immunologically competent cells. Further study of these cell populations may elucidate their contribution, if any, to pathologic conditions in the endometrium.