Borrelia burgdorferi, the spirochetal agent of Lyme disease, is susceptible to killing by a variety of polymorphonuclear leukocyte (PMN) components. Some are most effective against metabolically active B. burgdorferi. The abundant PMN cytoplasmic protein calprotectin, elevated 10- to 100-fold in inflammation, inhibits the growth of spirochetes through chelation of the essential cation, Zn. Since the action of some therapeutic antibiotics depends on bacterial division, we investigated the antibiotic sensitivities of spirochetes in calprotectin. In physiologic calprotectin, B. burgdorferi is not eliminated by therapeutic doses of penicillin G; in contrast, doxycycline is effective. Calprotectin may modify the clearance of spirochetes at sites of inflammation.