The diacylated lipopeptide FSL-1 enhanced the generation of IgG antibodies in TLR2^+/+ mice, but not in TLR2^−/− mice, when administered together with hen egg lysozyme as an antigen. Escherichia coli lipopolysaccharide enhanced the generation of antigen-specific antibodies in both TLR2^−/− and TLR2^+/+ mice. In TLR2^+/+ mice, the level of enhancement due to FSL-1 was similar to that caused by lipopolysaccharide. Analysis of the IgG antibodies subclass demonstrated that the level of Th2-type IgG1 antibodies was higher than that of Th1-type IgG2a antibodies. Both FSL-1 and lipopolysaccharide induced production of IL-10 and IL-6 by splenocytes from TLR2^+/+ mice. Lipopolysaccharide also induced production of these cytokines by splenocytes from TLR2^−/− mice, but FSL-1 did not. Neither FSL-1 nor lipopolysaccharide induced IL-12p70 production by splenocytes from either type of mice. FSL-1 upregulated B7.2 expression in B220⁺ cells from TLR2^+/+ mice but not those from TLR2^−/− mice, whereas lipopolysaccharide upregulated B7.2 expression in B220⁺ cells from both types of mice. FSL-1 and, to a lesser extent, lipopolysaccharide activated mitogen-activated protein kinases in splenocytes. FSL-1 and, to a lesser extent, lipopolysaccharide induced the expression of c-Fos, which is known to be involved in Th2-type responses, in splenocytes. Thus, this study demonstrated that FSL-1 possessed TLR2-mediated Th2-type responses in vivo.