We established two monoclonal antibodies (mAbs) which specifically recognize human 'functionally inactive' recombinant IL-18, and IL-18 protein polymorphism was examined using human monocytes and macrophages (M phi). In 6 day GM-CSF-treated M phi, an 'inactive" IL-18-recognizing mAb 21 detected the IL-18 preform (24 kDa) and a 48-kDa protein, which were gradually increased concomitant with maturation stage. Majority of the 24- and 48-kDa forms were barely detectable with other mAbs recognizing 'active' IL-18, No reagents including Toll stimulators upregulated these IL-18 populations in M phi. The 21-recognizable IL-18 species were separated using an anion-exchanger column and their IFN gamma -inducing activity was assessed with human lymphocytes plus IL-12, Virtually no as yet known activity was detected with these IL-18 species. After processed with M phi proteases, an 18-kDa form was generated to express the IFN gamma -inducing activity, although the activity was far weaker than that of control 'active' IL-18, These observations suggested that large amounts of various IL-18 species are produced with monocyte-M phi differentiation and most of these IL-18 species are functionally 'inactive' in terms of the reported IL-18 function even after proteolytic 18-kDa conversion.