A monoclonal antibody (mAb) to sheep red blood cells (SRBC), termed L180/1, is described that completely blocks rosette formation between SRBC and human or sheep T lymphocytes. L180/1 precipitated a minor glycoprotein of about approximately 42,000 mol wt from surface-labeled SRBC. This glycoprotein was partially affinity purified and found to block E rosette formation and to compete with anti-T11 mAb for the E receptor. The molecule detected by mAb L180/1 thus appears to be recognized by the E receptor and was given the preliminary name, T11 target structure (T11TS). Since the mAb to sheep T11TS blocks the binding of SRBC to both human and sheep T cells, and mAb to T11 blocks the binding of red cells from human and sheep to the human E receptor, we concluded that analogous receptor-ligand (T11-T11TS) systems exist in man and sheep that are crossreactive over the species barrier. The possibility is discussed that the E receptor, which is known to be involved in T cell activation, and T11TS function as complementary cell interaction molecules in T cell responses.