The MRL-lpr murine model of systemic lupus erythematosus (SLE) has provided many insights into the pathology of human lupus. The model is characterized by an age-dependent expansion of a Thy-1+ alpha beta/CD3+ CD4-, CD8- T-cell subset in the nodes and spleen. In this study, a lpr T-cell specific monoclonal antibody, Ye19.1, was found to bind to a 200 kDa cell surface molecule (termed LTA) which has a phosphotyrosine phosphatase (PTPase) enzymatic function. The significance of this marker in the development of autoimmune pathology in MRL/lpr mice was also demonstrated; treatment of MRL-lpr mice with the Ye19.1 Ab was shown to retard the development of the autoimmune syndrome and to restore the T cell-dependent immune response to ovalbumin.