Although the physiological role of α1-acid glycoprotein (AGP), an acute-phase protein, is poorly understood, several lines of evidence support a modulatory action on the immune response. In this study, we investigated the effect of AGP on the production of interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α by human monocytes, macrophages and the monocytic THP-1 cell line. AGP significantly enhanced (2- to 7-fold) the production of these cytokines in monocytes induced by suboptimal concentrations of lipopolysaccharide [E. coli lipopolysaccharide (LPS): 100 ng/ml] in serum-free conditions, whereas it had little or no effect in the absence of LPS. The potentiating effect of AGP was inhibited by specific antibodies. It was concentration dependent and the greatest enhancement was observed with 250–500 μg/ml. Moreover, AGP only potentiated the effect of suboptimal concentrations of LPS. AGP did not alter the time course of LPS-induced IL-1β, IL-6 or TNF-α secretion. AGP acts as a co-inducer and could also potentiate cytokine secretion triggered by Neisseria meningitidis LPS and muramyl dipeptide. The glycan moiety of AGP did not seem to be involved in its potentiating effect, since both its major glycoforms and asialo-AGP potentiated the effect of LPS to the same extent as native AGP. Possible differences in the effect of AGP according to cell maturation were investigated using isolated human macrophages: AGP potentiated LPS-induced cytokine production by both peritoneal and alveolar macrophages. These data suggest that AGP can modulate monocyte/macrophage functions, thereby contributing to the amplification and regulation of immune and inflammatory responses.