Female BALB/c mice were infested four times with pathogen-free Ixodes scapularis nymphs prior to infestation with nymphs infected with Borrelia burgdorferi B31. Each infestation was separated by a 14-day tick-free period. Mean weights of fed ticks and percentage reaching repletion did not indicate development of acquired resistance. Only 16.7% of mice repeatedly infested with pathogen-free ticks prior to infected I. scapularis nymph challenge became positive for B. burgdorferi. One hundred percent of control mice infested only with infected ticks were culture positive for B. burgdorferi.