Anti-rat CD5 monoclonal antibody recognizes a determinant expressed on all rat thymocytes and peripheral T lymphocytes. It has been determined that there are 15,000 antigenic sites for ABIN133204per cell on thymocytes and 30,000 on peripheral T cells. This antibody does not bind to B cells, macrophages, NK cells or mast cells (2). Also, no cross- reactivity has been detected with non-lymphoid cells in intestine, thymus, spleen or lymph node (2). The antigen recognized by ABIN133204is a surface glycoprotein of Mr 69,000 (2), and is most likely the homologue of the mouse Ly 1 and human CD5 antigen. The antigen has also been shown to have an important functional role in T cell activation and differentiation indicating a use for this antibody in related studies. The antibody may be employed to label cells with anti-mouse Ig conjugated to fluorescein, peroxidase or 125 and has been successfully used in labelling of cryostat sections (3). This clone is reported to work with frozen and paraffin sections (4).