Cytokines play an important role in the immune response induced by organ grafting, particularly during episodes of rejection. We tested the influence of monoclonal and polyclonal antibodies upon levels of mediators of the immune system. In 29 patients various cytokines and mediators were serially analyzed following transplantation. Thirteen patients received polyclonal antibodies (ATG) and 7 monoclonal (OKT3). Both OKT3 and ATG induced a rise in body temperature. Mean serum levels of amyloid A, neopterin, plasma levels of TNF alpha, interleukin 2 receptor (IL-2R) and urine levels of interleukin 6 (IL-6) and IL-2R were elevated when antibodies were employed. Interestingly, urine and plasma TNF-alpha as well as urine IL-6 and IL-2R remained elevated following ATG but not OKT3 and did not rise in case of basic treatment. In summary, antibody therapy increased levels of immune mediators. These mediators remained elevated following the discontinuation of treatment in case of ATG but not OKT3.