The antibody and complement requirements for killing of Borrelia burgdorferi 297 by normal human serum (NHS) and NHS plus immunoglobulin G (IgG) were examined. B. burgdorferi activated both the alternative and classical complement pathways in NHS. In NHS chelated with 10 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid plus 4 mM MgCl2 (Mg-EGTA) to block classical pathway activation, consumption (activation) of total hemolytic complement, complement component 3 (C3), and C9 by B. burgdorferi was observed. Furthermore, challenge of unchelated NHS with 297 cells resulted in the consumption of C4, in addition to an increase in C3 and C9 consumption over that observed in chelated serum. In spite of complement activation, B. burgdorferi was resistant to the nonspecific bactericidal activity of NHS. The addition of human anti-B. burgdorferi IgG to NHS, however, resulted in the complete killing of 297 cells. Bactericidal activity of this serum was abrogated if NHS was immunochemically depleted of C1, indicating that killing was mediated by the classical pathway. The manifestation of bactericidal activity was accompanied by a large increase in total complement and C3 consumption over that observed in NHS alone. Under similar conditions, only a minimal increase in C9 consumption was observed. No increase in total complement consumption was observed if NHS plus anti-B. burgdorferi IgG was treated with Mg-EGTA prior to challenge. The results of these experiments demonstrate that B. burgdorferi is resistant to the nonspecific bactericidal activity of NHS, in spite of classical and alternative complement pathway activation. B. burgdorferi is sensitive to serum, however, in the presence of IgG, which mediates bacterial killing through the classical complement pathway.