Background. Thymoglobulin®, a preparation of IgG purified from the sera of rabbits immunized with human thymocytes, is FDA approved for the treatment of transplant rejection. The perceived mechanism of action of Thymoglobulin® centers around T cell depletion which enables prolongation of graft survival. However, while Thymoglobulin® clearly effects T cell depletion, it also exhibits a portfolio of immunoregulatory activities that are not based on T cell reactivity yet contribute significantly to its efficacy. Examples include reactivities to adhesion molecules on vascular endothelial cells that inhibit T cell trafficking and to costimulatory determinants on antigen presenting cells that blunts T cell activation respectively (Bonnefoy-Berard et al., Transpl. 1991; Blood 1992; Immunol 1992; J Heart Lung Tx 1996; Rebellato, Transpl 1994; Bourdage Transpl 1995; Michallet Transpl 2003). Aims Bourdage and Hamlin (Transpl 1995) and Zand et al (ASCO, 2004) showed that Thymoglobulin® specifically binds to and induces apoptosis in human B cell lines and myeloma cells. We sought to 1) confirm and extend these findings to non-malignant human B and plasma cells and 2) identify the B cell reactive antibody specificities that reside within Thymoglobulin. Results Thymoglobulin® expresses complement independent, antibody dose dependent cytotoxicity for B cells and myeloma cells at clinically relevant Thymoglobulin® concentrations (effective Thymoglobulin® concentrations in vitro 10 - 100 mg/ml). In addition, Thymoglobulin®, at concentrations 10 – 100 mg/ml, was shown to inhibit human bone marrow derived plasma cell antibody production in vitro in a complement independent manner. Visual observation of Thymoglobulin® containing bone marrow cell cultures showed cell aggregation that was antibody associated. Thymoglobulin® contains antibody reactivities that appear to be B cell selective, as it retains the B cell reactivity even after exhaustive adsorption vs. human T cell antigens. T cell depleted B cell reactive antibodies were shown to significantly inhibit constitutive IgA and IgG production from antibody producing cells within normal human bone marrow cells in vitro. The mechanism by which Thymoglobulin® kills B cells and plasma cells in vitro is unknown, but did not require addition of complement. The nature of the B cell antigens recognized by Thymoglobulin® will be discussed.