Mice were infected with Borrelia burgdorferi by infection via Ixodes ricinus and experimental inoculation to determine whether transmission rates of spirochetes and antibody responses are influenced. Mice infected by the natural route were substantially more infective for ticks; two- to sixfold more tick larvae were positive for B. burgdorferi than those fed on experimentally inoculated mice. In natural infection, spirochetemia may be greater or spirochetes may be more accessible for transmission. Thus, this form of xenodiagnosis could be used to determine levels of spirochetes in the vertebrate host. Similar levels of antibody were present in all mice; however, those infected by the natural route lacked antibodies to outer surface proteins (Osp) A and B. The small antigen dose given through a tick bite may not have been sufficient to induce rapid OspA or OspB antibodies, thereby allowing the later development of higher levels of spirochetemia.