The adhesion receptor CD226 (DNAM-1) is a member of the Ig superfamily possessing two extracellular V-like domains. In humans, CD226 was shown to be expressed by NK as well as T cells. During T cell priming, CD226-mediated costimulatory signals may skew the subsequent differentiation into the Th1 pathway. In addition, CD226 expressed on NK and cytotoxic T cells is engaged by its counter-receptor CD155, present on target cells, thereby triggering their elimination. We established mAb specifically recognizing mCD226, demonstrating that CD226 is expressed by precursor and mature but not developing T cells. In contrast, NK cells are distinguished by a rather heterogeneous CD226 expression profile. In addition, expression of CD226 appears coupled to that of other NK cell receptors, as high expression of CD226 was found to correlate with decreased proportions of Ly49D and H positive NK cells. Upon injection into mice, the anti-CD226 antibodies caused selective depletion of CD8(+) T cells. Moreover, these antibodies as well as a naturally occurring CD226 splice variant lacking the outermost V-like domain were instrumental in determining that CD226 adheres to CD155 via its first domain. In addition, antibodies were identified as capable of blocking the CD226/CD155 interaction and to prevent NK-driven killing of immature DC. CD226 is thus the first mNK receptor identified to be essential for the elimination of this particular cell type.