Although much is known about classic IFNgamma inducers, little is known about the IFNgamma inducing capability of inflammasome-activated monocytes. In this study, supernatants from LPS/ATP-stimulated human monocytes were analyzed for their ability to induce IFNgamma production by KG-1 cells. Unexpectedly, monocyte-derived IFN inducing activity was detected, but it was completely inhibited by IL-1beta, not IL-18 blockade. Moreover, size-fractionation of the monocyte conditioned media dramatically reduced the IFNgamma inducing activity of IL-1beta, suggesting that IL-1beta requires a cofactor to induce IFNgamma production in KG-1 cells. Because TNFalpha is known to synergize with IL-1beta for various gene products, it was studied as the putative IL-1beta synergizing factor. Although recombinant TNFalpha (rTNFalpha) alone had no IFNgamma inducing activity, neutralization of TNFalpha in the monocyte conditioned media inhibited the IFNgamma inducing activity. Furthermore, rTNFalpha restored the IFNgamma inducing activity of the size-fractionated IL-1beta. Finally, rTNFalpha synergized with rIL-1beta, as well as with rIL-1alpha and rIL-18, for KG-1 IFNgamma release. These studies demonstrate a synergistic role between TNFalpha and IL-1 family members in the induction of IFNgamma production and give caution to interpretations of KG-1 functional assays designed to detect functional IL-18.