The potent immunosuppressive action of rabbit antithymocyte globulin (RATG) in allotransplant recipients has been recognized for many years. Some of the antibody specificities and immunoregulatory effects of RATG have been described, but a comprehensive definition of RATG components has not been reported previously. In this study, we have identified 23 specificities that are consistent among different clinical RATG batches and represent the major antibody specificities in RATG. These specificities were defined by immunoprecipitation/gel electrophoresis and also antibody blocking/flow cytometry methods. Titration studies performed for semiquantitative analysis of RATG antibodies showed that the antibodies present in highest titer were directed to CD6, CD16, CD18, CD28, CD38, CD40, and CD58 (titer > 1:4000), most of which are not T cell-specific antibodies. In contrast, the RATG antibodies that persisted the longest in vivo in the plasma of rhesus monkeys transplant recipients are antibodies to CD3, CD4, CD8, CD11a, CD40, CD45, CD54, and class I. These antibodies, which are directed at signal transduction and adhesion molecules, were present during the early period of lymphocyte recovery. We suggest that the persistence of these antibodies in vivo is directly related to the prolonged anergy of circulating T cells after RATG treatment and to the unusual potency and complex tapestry of immunological effects in transplantation.