Monoclonal antibodies reactive with distinct T lymphocyte subpopulations have been described in man, mouse and rat and structural analyses of these antigens have demonstrated a high degree of evolutionary conservation. This report describes the reactivity of three monoclonal antibodies (mAb), 19–19, αSBU‐T4 and αSBU‐T8, which define T cell subpopulations in the sheep. The mAb αSBU‐T4 and αSBU‐T8 define the sheep CD4 and CD8 molecules, respectively. These two antigens show similar tissue distributions, molecular weights and fluorescence‐activated cell sorter profiles to human, mouse and rat CD4 and CD8 molecules. The mAb 19–19 is reactive with a subpopulation of T lymphocytes which displays a tissue distribution unlike that reported for a T cell subset in any other species. 19–19 stains 7% of efferent lymph lymphocytes, 15% of peripheral blood lymphocytes but only 1–3% of lymph node lymphocytes. Two‐color immunofluorescence demonstrates that the 19–19+ T cell subset is SBU‐T4− and SBU‐T8−, and thus defines a third T cell subpopulation in sheep. Immunohistology on frozen lymph node tissue sections localizes 19–19 mAb‐reactive cells to the subcapsular region of the lymph node and lymph node trabeculae. Only 1% of thymocytes are 19–19+ and these cells are located mainly in the medulla and often arranged as foci around blood vessels. The 19–19 mAb immunoprecipitates from sheep lymphocytes an antigen with an apparent molecular mass of 215 kDa under both reducing and nonreducing conditions. It is concluded that αSBU‐T4 and αSBU‐T8 recognize the sheep homologues of the human T4 and T8 antigens, respectively, whereas 19–19 recognizes an antigen (termed SBU‐T19) which has not been reported in any other species.