We observed that high concentrations (up to 25% vol/vol) of serum (before or after vaccination) inhibited the NK cell CD25 response to rIL-18 (as expected) but had rather little effect on the response to cytokine cocktails (e.g., rIL-18 plus rIL-12) or EBOV GP, suggesting that while IL-18BP may limit the effects of IL-18, it may have less impact on the much lower synergistic combinations of cytokines induced by, for example, a viral infection or on the cell contact–mediated events at the NK cell–monocyte synapse. This evidence concerns the gene IL18BP and viral infectious disease.