Activated leukocyte cell adhesion molecule (ALCAM)/cluster of differentiation (CD166) is a type I transmembrane cell adhesion molecule belonging to the Ig superfamily and a ligand for CD6 that is expressed on T lymphocytes. Recently, homophilic (ALCAM-ALCAM) adhesion was shown to play important roles in tight cell-to-cell interaction and regulation of stem cell differentiation. To investigate the involvement of ALCAM in embryo implantation, the expression of ALCAM was examined in human blastocysts and endometrium. Immunohistochemical study showed that ALCAM was expressed on endometrial luminal and glandular epithelial cells but not on the endometrial stromal cells in either the proliferative or secretory phase. Northern blot analysis of isolated endometrial epithelial cells and stromal cells showed that ALCAM mRNA was expressed in endometrial epithelial cells. Flow cytometry confirmed cell surface expression of ALCAM on endometrial epithelial cells. On the other hand, nested RT-PCR analysis demonstrated that ALCAM mRNA was expressed in human blastocysts but not in the embryos in the 8-cell or morula stages, which were obtained from patients undergoing in vitro fertilization treatment. These findings indicate that ALCAM is expressed on human endometrial epithelial cells and blastocysts. The developing stage-specific expression on the embryo suggests that the ALCAM-ALCAM cell adhesion system is involved in an initial interaction of the embryo with maternal endometrium.