The cluster of differentiation 44 (CD44) Ag system, originally described in brain and mature T cells, has been subsequently shown to be identical with the human lymphocyte homing-receptor defined by the Hermes-1 antibody and to be involved in T cell/endothelial cell interactions in synovium, mucosa, and lymph node. CD44 is also present on human E. On E, CD44 has been shown to be regulated by the In(Lu) dominant inhibitor gene and to express the Ina and Inb blood group Ag. Because human E have been shown to interact with human T cells via CD2 on T cells and LFA-3 on human E, we have studied the ability of human E and T lymphocyte CD44 Ag to participate in CD2/LFA-3 interactions between human E and T cells. In this study, we demonstrate that a mAb (A3D8) against the CD44, In(Lu)-related p80, lymphocyte homing-receptor molecule inhibited the binding of human E to human T cells. Whereas whole CD44 antibody molecules inhibited human E binding to T cells, saturating amounts of CD44 Fab fragments did not inhibit human E to T cell binding. Our data demonstrated that anti-CD44 antibody A3D8 acted at the level of the E to inhibit CD2/LFA-3 interactions between human E and T cells.