230 Background: Antithymocyte globulins were introduced as potent immunosuppressive agents in organ transplantation in the late 1960s. The rabbit polyclonal IgG, Thymoglobuline® (IMTIX-Sangstat, France), acts mainly on T-lymphocytes and leads to a profound lymphopenia when used at the recommended arbitrary dose of 1.5-2.5 mg/kg. Our in vitro results suggest that low doses of Thymoglobuline® aimed at selectively depleting activated T cells in vivo while avoiding massive lymphocyte depletion could be clinically evaluated. Methods: Experiments were conducted in cynomolgus monkeys to investigate the dose effect of Thymoglobuline® (1 vs 5 mg/kg iv, days - 1,0,2,4,6,8,10,12,13,15,17,20) on MHC-mismatched skin and heterotopic heart transplantations. In parallel, the serum antibody content, the coating of monkey lymphocytes by rabbit polyclonal IgG, as well as the Thymoglobuline®-induced down-modulation of CD2, CD3, CD4 and CD8 were followed by FACS analysis using polyclonal FITC-conjugated Donkey Anti Rabbit IgG (DARIG-FITC) and the corresponding FITC- or PE- conjugated monoclonal antiCD, respectively. Results: These experiments demonstrated a dose effect of Thymoglobuline® on lymphocyte depletion, down-modulation of T-cell surface antigen and graft survival. The high dose regimen (5 mg/kg/d) induced massive lymphocyte depletion and free antibody excess in plasma. The low dose (1 mg/kg/d) resulted in moderate lymphocytopenia, very low levels of free antibody in serum and progressive increase in cell bound antibodies with a downregulation of surface expression of CD2, CD3, CD4 and CD8 on blood Tcells. A strong anti-rabbit IgG antibody response occurred in all animals with a subsequent major drop in serum Thymoglobuline® levels and cell bound antibodies. Nevertheless skin and heart allograft survival was markedly extended in the high dose group (22 vs. 8 days) and skin but not heart survival was moderately increased in the low dose group (13 vs. 8 days). Pharmacokinetic analysis of antibody levels, cell depletion and modulation of cell surface antigens will allow adjustments of Thymoglobuline® dosing and facilitate optimization of ATG therapy.