AF12198 is a potent, selective and specific peptide antagonist for human type I interleukin-1 receptor (IL1-R1) (IC50=8 nM) but not the human type II receptor (IC50=6.7 µM) or the murine type I receptor (IC50>200 µM). AF12198 inhibits IL-1-induced IL-8 production (IC50=25 nM) and IL-1-induced intercellular adhesion molecule-1 (ICAM-1) expression (IC50=9 nM) in vitro. AF12198 has anti-inflammatory activities and blocks responses to IL-1 in vivo.