DNA circulates in the blood in systemic lupus erythematosus, among other conditions, and plays a role in immunopathogenesis in the form of immune complexes. As shown in experiments in mice, blood DNA levels rise following treatments to induce apoptosis and the administration of cells made apoptotic or necrotic in vitro. In mice lacking macrophage function, however, blood levels do not rise following administration of dead cells. These results indicate that circulating DNA may be a marker of cell death, although its levels likely reflect a complex process involving the interactions of macrophages with dead and dying cells.