The effects of gangliosides on human B-cell responses were studied. Of various gangliosides tested, only GM2 and GM3 inhibited production of IgG subclasses and IgM, but not IgA subclasses, and thymidine uptake by human B cells stimulated with SAC plus interleukin-2 (IL-2). In contrast, GM1, GD1a, GD1b, GD3, GT1b, and GQ1b were without effects. GM2- and GM3-induced inhibition were specific, because each was blocked by a corresponding antibody. Of various cytokines tested, tumor necrosis factor-alpha (TNF-alpha) alone counteracted GM2- and GM3- induced inhibitions of Ig production and thymidine uptake, whereas other cytokines including IL-1 beta, IL-3, IL-5, IL-6, and interferon- gamma each failed to do so. Moreover, anti-TNF-alpha antibody, but not control IgG, blocked the counteraction of inhibition by TNF-alpha. GM2 and GM3 each inhibited Ig production, thymidine uptake, and TNF-alpha production by surface IgG1+ (slG1+), sIgG2+, sIgG3+, sIgG4+, and sIgM+ B cells without affecting IL-2 binding or TNF-alpha binding to B cells, but had no such inhibitory effects on sIgA1+ or sIgA2+ B cells. These findings indicate that GM2 and GM3 inhibit Ig production and thymidine uptake by human sIgG1+, sIgG2+, sIgG3+, sIgG4+, and sIgM+ B cells by inhibiting endogenous TNF-alpha production.