The adaptive immune system can generate distinct classes of responses, but the mechanisms that determine this are poorly understood. In this study, we demonstrate that different Toll-like receptor (TLR) ligands induce distinct dendritic cell (DC) activation and immune responses in vivo. Thus, Escherichia coli LPS (TLR-4 stimulus), activates DCs to produce abundant IL-12 (p70), but little IL-10, and stimulates Th1 and Tc1 responses. In contrast, Pam-3-cys (TLR-2 stimulus) elicits less IL-12 (p70), but abundant IL-10, and favors Th2 and T cytotoxic 2 (Tc2) responses. These distinct responses likely occur via differences in extracellular signal-regulated kinase signaling in DCs. Thus, Pam-3-cys induces enhanced extracellular signal-regulated kinase signaling, compared with LPS, resulting in suppressed IL-12 (p70) and enhanced IL-10 production, as well as enhanced induction of the transcription factor, c-Fos. Interestingly, DCs from c-fos−/− mice produce more IL-12 (p70), but less IL-10, compared with control DCs. Therefore, different TLR ligands induce distinct cytokines and signaling in DCs, and differentially bias Th responses in vivo.