Rheumatoid arthritis (RA) is a chronic illness, immunologically mediated, probably induced by the exposure to an antigen or antigens, to which immunologic tolerance is lost. The disease has a variable course, from a mild, intermittently symptomatic illness requiring only symptomatic therapy to a fulminant illness requiring dangerous immunosuppressive therapy, surgery or both. The molecular defect causing RA has not been characterized, but may involve aberrant T cell, B cell, and macrophage function. Although RA often responds to immunosuppressive medication including corticosteroids, methotrexate, azathioprine and cyclophosphamide, or to non-steroidal anti-inflammatory drugs, no therapy has been curative. In patients with severe RA, who have been unresponsive to corticosteroids, and who have more than 20 active joints or vasculitis, we propose, as a phase I-II study, complete immune ablation and subsequent reconstitution with autologous in vitro T lymphocyte depleted PBSCs harvested from the patient prior to immune ablation. The combination of high dose cyclophosphamide and anti-thymocyte globulin conditioning will be followed by rescue with autologous lymphocyte depleted PBSCs. Subsequent disease activity will be followed by: (1) RA disease activity index, (2) type and amount of therapy for RA, and (3) flow cytometry of peripheral blood lymphocyte subsets, (4) joint count, (5) patients' assessment of pain, (6) arthritis impact measurement scales (AIMS) questionnaire, (7) acute phase reactants. This study will dose standard therapy, i.e. immune suppression, to the point of complete immune ablation and subsequent recapitulation of lymphocyte ontogeny by PBSC rescue. We anticipate that this study will also form the basis to clarify further the role of the immune system in RA.