Cell surface components of pathogens, such as lipopolysaccharide (LPS), are an important signal for receptor‐mediated activation of immune cells. Here we demonstrate that DNA of gram‐positive and gram‐negative bacteria or certain synthetic oligonucleotides displaying unmethylated CpG‐motifs can trigger macrophages in vitro to induce nuclear translocation of nuclear factor‐xB, accumulate tumor necrosis factor (TNF)‐α mRNA and release large amounts of TNF‐α. In vivo these events culminate in acute cytokine‐release syndrome which includes systemic but transient accumulation of TNF‐α. D‐Galactosamine (D‐GalN)‐sensitized mice succumb to lethal toxic shock due to macrophage‐derived TNF‐α resulting in fulminant apoptosis of liver cells. LPS and a specific oligonucleotide synergized in vivo as measured by TNF‐α‐release, suggesting that macrophages integrate the respective signals. The ability of macrophages to discriminate and to respond to bacterial DNA with acute release of pro‐inflammatory cytokines may point out an important and as yet unappreciated sensing mechanism for foreign DNA.