The interleukin-1 receptor antagonist (IL-1ra) is a protein capable of inhibiting receptor binding and biological activities of IL-1 without inducing an IL-1-like response. Equilibrium binding and kinetic experiments show that IL-1ra binds to the 80-kDa IL-1 receptor on the murine thymoma cell line EL4 with an affinity (KD = 150 pM) approximately equal to that of IL-1 alpha and IL-1 beta for this receptor. However, IL-1ra is unable to induce two early events associated with IL-1 activity. Surface-bound IL-1ra does not undergo receptor-mediated internalization, and IL-1ra does not activate the protein kinase activity responsible for down-modulation of the EGF receptor on the murine 3T3 fibroblast cell line. The failure to induce general, early responses characteristic of IL-1 indicates that IL-1ra is unlikely to act as an agonist on any cell expressing the 80-kDa receptor.