Gangliosides are sialic acid-containing glycolipids and have various immunomodulatory effects. We previously reported that various gangliosides in vitro either inhibited or enhanced spontaneous immunoglobulin production by human peripheral blood mononuclear cells (PBMC). Among them, GT1b was the most inhibitory. In this study, we further examined the mechanism for the inhibitory effect of GT1b. The inhibitory effect of GT1b was apparent at 0.1 micrometers, increased dose dependently, and was maximal at 10 micrometers. In the presence of 10 micrometers GT1b, spontaneous production of immunoglobulin (Ig)G, IgM and IgA in human PBMC was reduced by 60%, 59.5% and 58%, respectively, compared with controls. GT1b did not affect the proliferation and viability of PBMC, and did not enhance their apoptosis. GT1b did not alter immunoglobulin production of B cells alone. Interleukin (IL)-6 and IL-10 each partially reversed the GT1b-induced inhibition of immunoglobulin production by PBMC, and the presence of both cytokines completely reversed the inhibition. GT1b inhibited IL-6 and IL-10 production in monocytes, without affecting that in T or B cells. When monocytes were preincubated with GT1b, washed and then cultured with B and T cells, the immunoglobulin production was also suppressed. These results suggest that GT1b may indirectly suppress immunoglobulin production of B cells in whole PBMC via reducing the production of IL-6 and IL-10 in monocytes. It is thus indicated that GT1b may act as an important inhibitor for human humoral immune responses.