Inflammation induces the hepatic synthesis of the iron storage protein (apo)ferritin, which is released into the circulation, and behaves as an acute phase protein. The biological significance of the extracellular rise in serum (apo)ferritin is unknown. We have observed that (apo)ferritin will stimulate superoxide production from neutrophils in the presence of cytochalasin B across a physiologically appropriate concentration range. We therefore propose that extracellular ferritin has an important role in host defence against bacteraemia by stimulating oxidative metabolism.