Background.Polyclonal antithymocyte globulins (ATG) induce T-cell depletion and functional impairment of nondeleted lymphocytes. Interference of ATG with the main leukocyte surface molecules involved in cellular adhesion and leukocyte-endothelium interaction was investigated in the present study.Methods.In three rabbit ATG, the authors measured antibodies to integrins, &bgr;2-integrin ligands, and chemokine receptors by flow cytometry; chemotactic responses; and down-modulation of cell surface expression on lymphocytes, monocytes, and neutrophils.Results.Antibodies to CD11a/CD18 (leukocyte function-associated antigen-1 [LFA-1]) present in ATG induced a dose-dependent down-modulation of cell surface expression of this &bgr;2 integrin on lymphocytes, monocytes, and neutrophils. In contrast, anti–LFA-1 monoclonal antibodies did not induce LFA-1 modulation unless cross-linked by a second antibody. ATG also contained functional antibodies to the &bgr;1 integrin CD49d/CD29 (VLA-4), the &agr;4&bgr;7 integrin, CD50, CD54, and CD102 but not to CD62L. ATG were shown to bind to CXCR4 and CCR7 on lymphocytes, CXCR4, and CCR5 on monocytes; to down-modulate cell surface expression of CCR7; and to decrease monocyte chemotactic response to CCL5 (RANTES) and lymphocyte chemotactic response to CCL19 (MIP-3&bgr;).Conclusion.These results show that ATG may interfere with leukocyte responses to chemotactic signals but mostly inhibit the expression of integrins required for firm cellular adhesion. The latter property of inhibition is not shared by monoclonal antibodies, and it may contribute to decreasing graft cellular infiltration during acute rejection and possibly after postischemic reperfusion.