Monoclonal antibodies (mAb) to ovine T-lymphocyte molecules SBU-T8 (Maddox, Mackay & Brandon, 1985), the cytotoxic T lymphocyte, human CD8 equivalent, and SBU-T6 (Mackay et al., 1985), the immature T lymphocyte human CD1 equivalent, were reported to cross-react with bovine cells. We have confirmed this cross-reactivity and shown that the percentage of T lymphocytes in thymus, lymph node and peripheral blood is equivalent to that reported in sheep. In addition, we have shown that the molecular weight of the molecules identified by these antibodies (T8, 34,000-38,000; T6, 44,000 and 13,000) is similar to that reported in sheep and humans. The monoclonal antibody to SBU-T8 effectively removes bovine cytotoxic cells in the presence of complement and when used alone blocks killing of allogeneic concanavalin A (Con A) lymphoblasts by mixed lymphocyte-generated cytotoxic T lymphocytes. These results confirm that the anti-SBU-T8 antibody 38-65 binds BoCD8 and anti-SBU-T6 antibody 20-27 binds BoCD1 and that these reagents will be useful in the study of the bovine immune system.