Abstract A new monoclonal mouse antibody that recognizes a subset of rat peripheral T cells has been prepared by immunizing mice with rat thymocyte glycoprotein. This antibody, designated MRC OX 8, labels all peripheral T cells that are unlabeled by the previously described W3/25 monoclonal antibody. No peripheral T cells were found that bound both antibodies, but, in contrast, 90% of thymocytes were doubly labeled. Thoracic duct lymphocytes of congenitally athymic nude rats were not labeled by either antibody, but the spleens of such animals contained both W3/25 + cells and MRC OX 8 + cells. These splenocyte subpopulations did not overlap. Using the fluorescence‐activated cell sorter to isolate cells binding MRC OX 8 antibody, the phenotype of T cells mediating various T cell functions was established. Combining the present results with those published previously, it is shown that the cells providing help for antibody responses and those mediating graft‐ vs. ‐host reactions are phenotypically W3/25 + MRC OX 8 − . On the other hand, parental T cells that suppress antibody formation in F 1 hosts were identified as W3/25 − MRC OX 8 + . The relationship between the rat T cell subsets defined by these antibodies and those in the mouse identified by the Ly series of alloantibodies is discussed and a comparison made between the rat W3/25 + subset and a recently identified human T cell subset.