ABSTRACT: Emerging evidence suggests that cytokines play significant roles as intercellular communication signals in human endometrium. In the present report, an immunohistochemical staining method and tumor necrosis factor-α (TNF-α) specific polyclonal antibody was used to identify potential in vivo sources of this cytokine in human endometrium. During the proliferative and early secretory phases of the menstrual cycle, glandular epithelium was distinctly free of any immunoreactive product. With the emergence of mid- and particularly late-secretory phases, however, some glandular epithelial cells started to express TNF-α. In the late secretory phase, glands exhibiting strong staining were adjacent to those that were negative for TNF-α or only sparingly contained positively immunostained cells. Remarkably, the glandular epithelium constituted a major source of enhanced expression of TNF-α during gestation. In the proliferative phase, virtually all stromal cells strongly expressed TNF-α, and in the secretory phase, some variability of staining could be observed among various stromal cells. In gestational endometria, decidual cells and in all endometria, endothelial cells strongly expressed immunoreactivity for TNF-α. These data demonstrate that various constituents of endometria are constitutively primed to express TNF-α and that this expression is distinctly associated with the changes in endometrium that are compelled by hormonal stimuli.