[Aims] TLR4/MD-2 is an essential complex for LPS recognition. RP105/MD-1 is a homologue of TLR4/MD-2 and is expressed in mainly B cells. RP105 or MD-1-deficient B cells show impaired LPS responsiveness, suggesting that B cells require both TLR4/MD-2 and RP105/MD-1 signals for LPS responses. However, precise mechanisms that functionally couple with the two types of complex remain unclear. Furthermore, as LPS stimulation activates B cells through both receptors, it is difficult to analyze the signals via TLR4/MD-2 and RP105/MD-1 differentially. To resolve this, we used agonistic mAbs to TLR4 and RP105 to explore differential roles for TLR4/MD-2 and RP105/MD-1 in B cell responses to LPS. [Methods] Mouse splenic B cells were stimulated with αTLR4 mAb, αRP105 mAb, or LPS in combination with IL-4. Expression of CD86, BAFF receptors (TACI, BAFF-R, BMCA), and CD138 was analyzed by FACS. For proliferation analysis, the B cells were stimulated with vehicles for 3 days and pulsed with 0.5 μCi [3H]-TdR. The μ to γ1 class switch recombination (CSR) was examined by analyzing the proportion of surface IgG1+ cells by FACS. [Results and Discussion] The αTLR4 mAb and αRP105 mAb induced massive B cell proliferation like as LPS. Expression of CD86 and BAFF receptors was induced by low doses of αRP105 (10 ng/ml), whereas those expression was very low by αTLR4 stimulation even at high doses (1 μg/ml). Combination of αTLR4 + IL-4 as well as LPS + IL-4 forced B cells to induce sIgG1+ cells. In contrast, αRP105 + IL-4 never showed this activity. CD138 expression was induced on B cells stimulated with αTLR4 but not with αRP105. These findings suggest that TLR4 and RP105 differentially regulate B cell response to LPS.