A phase I study of human recombinant interleukin-1 receptor antagonist (IL-1ra) was conducted in healthy males between the ages of 18 and 30. Twenty-five volunteers received a single, 3 h continuous intravenous infusion of doses ranging between 1 mg/kg and 10 mg/kg IL-1ra. At 3 h into the infusion, plasma IL-1ra levels were 3.1 μg/ml and 29 μg/ml for the 1 mg/kg and 10 mg/kg doses, respectively. Post-infusion plasma IL-1ra levels declined rapidly, exhibiting an initial half-life of 21 min and a terminal half-life of 108 min. Clinical, hematological, biochemical, endocrinological and immunomodulatory effects were monitored over 72 h and compared to those of four subjects receiving a 3 h infusion of saline. There were no clinically significant differences between the drug and saline groups in symptoms, physical examinations, complete blood counts, mononuclear cell phenotypes, blood chemistry profiles, serum iron and serum cortisol levels. Peripheral blood mononuclear cells (PBMC) obtained after completion of the IL-1ra infusion synthesized significantly less interleukin 6 ex vivo than PBMC from saline-injected controls. These data suggest that transient blockade of interleukin 1 receptors is safe and does not significantly affect homeostasis.