To clarify the mechanism that causes elevation of plasma fibrinogen levels in diabetes, we examined the effect of high concentration of glucose and/or advanced glycosylation end products (AGEs) on the production of interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) by human monocytes. Monocytes isolated from nine healthy volunteers were incubated with glucose, glucose with mannitol, or glucose with AGE-BSA for 24 or 48 h, respectively. IL-6 and TNF-alpha levels of culture supernatants were measured by ELISA methods. IL-6 and TNF-alpha levels of culture supernatants incubated with 22 mM or 33 mM glucose showed considerable increase over basal levels incubated with 11 mM glucose, whereas those levels incubated with high concentration of mannitol showed no increase. These two cytokine levels of culture supernatants, especially IL-6 level, showed synergistic elevation with AGE-BSA concentration. Our serial observation with treatment for lowering glucose levels showed that the diabetics with decreasing plasma fibrinogen levels also showed decrease in plasma IL-6 levels. In this study, we revealed the effect of glucose and AGEs on the production of IL-6 or TNF-alpha by human monocytes. These results suggest that hyperglycemia and AGEs will cause disregulated production of IL-6 and hyperfibrinogenemia in diabetics.