The aim of this study was to clarify the role of phosphatidylinositol 3-kinase (PI3K) on the production of interleukine-10 (IL-10) and IL-12 in mouse peritoneal macrophages stimulated with CpG-ODN. CpG-ODN-induced IL-10 mRNA expression and protein production decreased following the treatment of macrophages with wortmannin and LY294002, specific inhibitors for PI3K. In contrast, IL-12 p40 mRNA expression and p70 protein production increased. Neutralizing anti-IL-10 monoclonal antibody (anti-IL-10 mAb) exactly mimicked the effects of PI3K inhibitors to enhance IL-12 p70 production. The enhancement effect of PI3K inhibitors on IL-12 p70 production almost completely disappeared by the treatment with anti-IL-10 mAb. PI3K inhibitors suppressed the activation of extracellular-regulated kinase (ERK), a member of the mitogen-activated protein kinases, by CpG-ODN. A specific ERK inhibitor, U0126, as well as PI3K inhibitors, differentially regulated IL-10 and IL-12 p70 productions. These results suggest that PI3K positively and negatively regulates the production of CpG-ODN-induced IL-10 and IL-12 p70, respectively, and negatively regulates IL-12 p70 production in macrophages through ERK-mediated IL-10 induction.