A massive and self-limited release of tumor necrosis factor and interferon gamma was detected in the systemic circulation in 35 consecutive renal allograft recipients by specific radioimmunoassays very soon following the first injection of the monoclonal antibody OKT3 (anti-CD3). Peak serum TNF and IFN [gamma] levels were reached, respectively, at 1 and 4 hr following the first OKT3 injection. Abnormally high serum interleukin 2 levels were also observed 4 hr following the first OKT3 injection in a minority of patients (5 cases). OKT3 had no effect on interleukin 1 beta, interferon alpha, and granulocyte/macrophage colony stimulating factor serum levels, which in all patients remained within the normal range throughout the study. This selective OKT3-induced cytokine release, which only followed the first injection, was transient (ie, lasting a few hours). It tightly paralleled the spontaneously reversible clinical syndrome characterized by high fever, headaches, and gastrointestinal symptoms that is invariably associated with the first OKT3 administration.