Abstract Interaction of B. burgdorferi organisms with mouse bone marrow-derived macrophages (BMMΦ) leads to phagocytosis of microorganisms, induction of nitric oxide (NO) and superoxide radicals (O 2 − by BMMΦ and killing of spirochetes. Destruction of spirochetes by BMMΦ was quantified by a new method based on the release of radioactivity from spirochetes pre-labelled with [ 3H]adenine. Uptake of B. burgdorferi by BMMΦ, which mainly occurs by coiling phagocytosis, generation of NO and O 2 − radicals as well as killing of spirochetes were significantly enhanced by pre-opsonization of spirochetes with monoclonal antibodies (mAb) to the outer surface proteins A and B but not with those to the periplasmic flagellin. Addition of inhibitors specific for NO and O 2 − radical synthesis either separately or together to cultures of BMMΦ and spirochetes resulted in only partial reduction of the killing potential of effector cells. The data indicate that NO and O 2 radicals are necessary, but not sufficient, for complete elimination of B. burgdorferi by macrophages. Together with previous findings that protection against B. burgdorferi infection is conveyed by humoral immune responses the present data indicate that one of the important functions of specific antibodies is their participation in macrophage-mediated control of spirochetes.