Bacterial DNA containing unmethylated cytosine-phosphate-guanosine motif (CpG-DNA) has been identified as a pathogen-associated molecular pattern, which is recognized by Toll-like receptors and activates immune cells to produce cytokines. Aim of the study was to characterize the ability of CpG-DNA to induce fever in mice. Intravenous administration of unmethylated CpG-DNA 1826 triggered an elevation of body temperature (T_b) lasting several hours. The magnitude of T_b elevation increased with an increase of dose of the oligonucleotide (administered in a range from 0.01 mg/kg to 1.0 mg/kg). A fever-like increase of T_b in mice was partially dependent on IL-6, since IL-6 deficient mice responded with reduced fever to the CpG-DNA 1826. Meloxicam and sulindac sulfide, inhibitors of cyclooxygenases, reduced fever in mice challenged with CpG-DNA 1826, indicating that the process may also depend on prostaglandins. In fact, plasma levels of prostaglandin E₂, as well as IL-6, increased at 4 h post-injection of CpG-DNA 1826 into mice. These data demonstrate that the pathophysiologic mechanism of the increase of T_b induced by CpG-DNA 1826 is similar to fever induced by lipopolysaccharide (LPS). Both LPS and CpG-DNA 1826 failed to produce elevation of T_b in mice deficient for a nuclear-factor-kappa-B (NFB) gene, further supporting the hypothesis that the two pyrogens provoke fever utilizing the same components of the cellular signaling metabolism. However, parthenolide, an inhibitor of I-B kinase reduced fever due to CpG-DNA 1826, and did not affect fever to LPS, suggesting that the two structurally dissimilar pyrogens may affect different intracellular pathways leading to the up-regulation of NFB. In support of this hypothesis, we demonstrate that C3H/HeJ mice known to exhibit a mutation in the Toll-like receptor-4 gene do not respond with fever to LPS. They respond, however, with fever following injection of CpG-DNA 1826. We conclude that bacterial DNA shares with components of the bacterial wall the capacity to elicit fever and may, consequently, be part of a novel class of exogenous pyrogens.