An immunohistochemical study of human endometrium has been carried out using a panel of monoclonal antibodies directed against T cells and their subsets, B cells, macrophages, natural killer cells, dendritic reticulum cells, human leucocyte antigen class II, leucocyte common antigen, and proliferating cells. Normal endometrium at different phases of the menstrual cycle and abnormal endometrium were examined and the number of cells of each type was counted. Throughout the menstrual cycle, T-lymphocytes and macrophages form a significant proportion of the normal stromal population (mean 4 and 5%, respectively). They are seen in basal aggregates, scattered through the stroma, located between epithelial cells, and in gland lumina. B cells are rare except in basal aggregates. In late secretory phase, large numbers of T cells of an unusual phenotype appear (mean 16% stromal cells). These bear early (OKT 11 and 3A1) but not mature (UCHT 1, OKT 4, and OKT 8) T cell markers. They correspond to stromal granulocytes, which are prominent in paraffin-embedded tissue at this phase of the cycle.