Addition of transforming growth factor-beta 1 (TGF-beta 1) to in vitro cultures of murine B cells activated with bacterial LPS selectively stimulates IgG2b and IgA class switching and decreases cellular proliferation. To assess a possible role for endogenous TGF-beta in modulating the Ig isotypes produced by LPS-activated cells, we utilized a neutralizing anti-TGF-beta mAb to abrogate endogenous TGF-beta activity. Anti-TGF-beta antibody, over a range of relatively low cell densities, strikingly inhibited both IgG3 and IgG2b production in response to LPS, with little or no change in the concentrations of secreted IgM. This effect of anti-TGF-beta antibody was specific, since it did not occur with an isotype-matched control mAb and was completely reversed with exogenous TGF-beta 1. Optimal IgG3 secretion occurred at concentrations of TGF-beta that were approximately eightfold lower than that necessary for maximal synthesis of IgG2b. Neutralization of endogenous TGF-beta in LPS-activated cultures was associated with an approximately twofold increase in proliferation and viable cell yields, a modest decrease in the percentage of membrane (m)IgG2b+ cells, and a modest increase in the percentage of mIgG3+ cells. This latter finding indicated that TGF-beta was not required for IgG3 class switching, but for maturation of mIgG3+ cells into Ig secretors. Highly purified B cells, obtained by electronic cell sorting, released active TGF-beta in response to LPS and showed a similar marked reduction in LPS-mediated IgG3 and IgG2b secretion in the presence of anti-TGF-beta antibody. Abrogation of endogenous TGF-beta activity in LPS-activated cultures also resulted in a striking reduction in IFN-gamma-mediated IgG2a production, and a more modest decrease in the synthesis of IgG1 and IgE in the presence of IL-4. These data indicate that relatively low concentrations of TGF-beta are essential for stimulating optimal IgG secretion by LPS-activated B cells, in an Ig isotype-nonspecific manner, and may regulate these responses in an autocrine fashion.