The present study was designed to investigate the effect of membrane proteoglycans (MPG) from Klebsiella pneumoniae on IL-6 production by human peripheral blood monocytes. Exposure in vitro to MPG induced release of IL-6 activity from human monocytes, as assessed by the 7TD1 hybridoma assay. MPG-induced hybridoma growth factor activity was blocked by anti-IL-6 antibodies. MPG induced expression in human monocytes of IL-6 mRNA transcripts as assessed by Northern blot analysis. Induction of IL-6 in mononuclear phagocytes may play a role in the immunomodulatory activity of MPG.