Background.Use of the murine CD3 monoclonal antibody OKT3 is limited by first-dose side effects, which are thought to be caused by the release of inflammatory mediators. Because these processes might be influenced by the speed of administration, we compared a 2-hr OKT3 infusion with the bolus infusion usually applied nowadays.Methods.Eighteen renal allograft recipients were prophylactically treated with OKT3 and randomized to receive the first dose either as a 2-hr infusion or as an intravenous bolus infusion. Clinical side effects score and the occurrence of complement activation, cytokine release, and activation of neutrophils were determined.Results.Two-hour infusion of OKT3 completely prevented the occurrence of dyspnea, reduced the incidence of other side effects, and attenuated complement activation. Cytokine release and depletion of peripheral blood lymphocytes were similar in both groups.Conclusions.Thus, complement activation seems to play an additional role in the development of side effects after the first OKT3 dose.