A novel, constitutively expressed and secreted IL-18 binding protein (IL-18BP) neutralizes IL-18 and thereby suppresses the production of IFN-γ, resulting in reduced T-helper type 1 immune responses. In the present study, four human and two mouse isoforms, resulting from mRNA splicing and found in various cDNA libraries, were expressed, purified, and assessed for binding and neutralization of IL-18 biological activities. Human IL-18BP isoform a (IL-18BPa) exhibited the greatest affinity for IL-18 with a rapid on-rate, a slow off-rate, and a dissociation constant (K_d) of 399 pM. IL-18BPc shares the Ig domain of IL-18BPa except for the 29 C-terminal amino acids; the K_d of IL-18BPc is 10-fold less (2.94 nM). Nevertheless, IL-18BPa and IL-18BPc neutralize IL-18 >95% at a molar excess of two. IL-18BPb and IL-18BPd isoforms lack a complete Ig domain and lack the ability to bind or neutralize IL-18. Murine IL-18BPc and IL-18BPd isoforms, possessing the identical Ig domain, also neutralize >95% murine IL-18 at a molar excess of two. However, murine IL-18BPd, which shares a common C-terminal motif with human IL-18BPa, also neutralizes human IL-18. Molecular modeling identified a large mixed electrostatic and hydrophobic binding site in the Ig domain of IL-18BP, which could account for its high affinity binding to the ligand. It is likely that preferential secretion of functional and nonfunctional isoforms of IL-18BP affect the immune response.