Borrelia burgdorferi grew more slowly in iron-depleted than in iron-sufficient media. The addition of increasing concentrations of iron stimulated borrelial growth and resulted in the intracellular accumulation of this element. Compared with iron-starved borrelia, iron-enriched organisms showed enhanced sensitivity to hydrogen peroxide. Intracellular iron-content did not, however, influence susceptibility to killing by human polymorphonuclear leucocytes [corrected].