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 ABIN133208per 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 ABIN133208is 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 125I and has been successfully used in labelling of cryostat sections (3).