Interleukin-1 receptor type I, IL-1 beta, IL-1 receptor antagonist, and human macrophages were immunohistochemically localized in the villous trophoblast, maternal-trophoblast interphase, and maternal decidua during early human implantation. Immunostaining for IL-1 receptor type I was present in the syncytiotrophoblast and hyperplastic endometrial glands in the maternal decidua. Immunoreactive IL-1 beta was present in the villous cytotrophoblast, syncytiotrophoblast, intermediate trophoblast, and maternal stromal decidual cells. IL-1 receptor antagonist staining was observed in the glandular endometrium of the maternal decidua and in isolated cells located inside the chorionic villi, intervillous space, and maternal decidua. Mature human macrophages, as defined by both CD/68+ and HAM56+, were present in the chorionic villi, maternal blood of intervillous space, and maternal decidua. Co-localization studies demonstrated that macrophages in all of the reported locations also stained for immunoreactive IL-1 beta. Our results show the shared presence in maternal and embryonic tissues of this receptor-agonist-antagonist system during early human implantation. This finding supports an autocrine/paracrine role for the IL-1 system in human implantation.