We have recently reported that a novel molecule, murine IFN-gamma-inducing factor (IGIF) produced by mouse liver cells, possesses potent biologic activities, including the induction of IFN-gamma production by spleen cells and the enhancement of NK cell cytotoxicity. In this paper, we report on the isolation of human IGIF cDNA clones from normal human liver cDNA libraries using murine IGIF cDNA as a probe. The amino acid sequence deduced from the human cDNA clones indicated a 193-amino acid precursor peptide and revealed 65% homology with that of murine IGIF. The amino acid sequence of IGIF also included an IL-1 signature-like sequence. Subsequently, the cloned cDNA was expressed in Escherichia coli, and preliminary studies on the biologic activities of the recombinant protein were performed. The recombinant human IGIF induced IFN-gamma production by mitogen-stimulated PBMC and enhanced NK cell cytotoxicity, in a manner similar to murine IGIF. In addition, recombinant human IGIF also augmented granulocyte-macrophage-CSF production and decreased IL-10 production, but had no effect on IL-4 production by Con A-stimulated PBMC. Based on these pleiotropic effects of IGIF, we propose that this novel cytokine be designated as IL-18.