Interleukin 18 (IL-18) is an essential cytokine for both innate and adaptive immunity. IL-18 signals through binding to IL-18 receptor alpha (IL-18R), which contains sequence homology to IL-1R and Toll-like receptors (TLRs). While the accessory protein AcP is required for IL-1R signaling, other accessory proteins have roles in regulating TLR signaling. An ACP-like molecule (AcPL) has been identified with the ability to cooperate with IL-18Ralpha in vitro, however, its physiological function remains unknown. Here we demonstrate that IL-18 signals are abolished in AcPL-deficient mice and cells. AcPL-deficient splenocytes fail to respond to IL-18-induced proliferation and interferon-gamma production. AcPL-deficient Th1 cells, dendritic cells and neutrophils also have impaired IL-18-induced activation. Furthermore, AcPL-deficient mice show impaired IL-18-induced NK cytotoxicity. AcPL, however, is dispensable for the activation or inhibition of IL-1R and the various TLR signals that we study. These results suggest that AcPL is a specific cell surface receptor essential for IL-18 signaling.