Lipopolysaccharide (LPS) is a major component of environmental microbial products. Studies have defined the LPS dose as a critical determining factor in driving differential T-cell polarization but the direct effects of LPS on individual antigen-presenting cells is unknown. Here, we investigated the effects of LPS doses on naive B cells and the subsequent modulatory effects of these LPS-activated B cells on T-cell polarization. The LPS was able to induce a proliferative response starting at a dose of 100 ng/ml and was capable of enhancing antigen internalization at a dose of 1 microg/ml in naive B cells. Following LPS stimulation, up-regulation of the surface markers CD40, CD86, I-Ad, immunoglobulin M, CD54 and interleukin-10 production, accompanied by down-regulation of CD5 and CD184 (CXCR4) were observed in a LPS dose-dependent manner. Low doses (<10 ng/ml) of LPS-activated B cells drove T helper type 2 polarization whereas high doses (>0.1 microg/ml) of LPS-activated B cells resulted in T regulatory type 1 cell polarization. In conclusion, LPS-activated B cells acquire differential modulatory effects on T-cell polarization. Such modulatory effects of B cells are dependent on the stimulation with LPS in a dose-dependent manner. These observations may provide one of the mechanistic explanations for the influence of environmental microbes on the development of allergic diseases.