Lyme disease spirochetes (Borrelia burgdorferi) are naturally maintained in an enzootic cycle mainly by vector ticks (Ixodes dammini) infesting white-footed mice (Peromyscus leucopus). Suggestions that mice may become infected without exposure to ticks prompted a study to evaluate whether mice could transmit spirochetes transplacentally to their offspring. Mice were live-captured in two Massachusetts sites where Lyme disease spirochetes are intensely enzootic. Pregnant females were housed separately in the laboratory through delivery, and mothers and their offspring were caged together until weaning. Each female and two offspring were then examined for evidence of infection serologically and by tick xenodiagnosis. All 14 mother mice examined produced infected ticks and exhibited serum antibodies to B. burgdorferi. However, none of 28 offspring tested produced infected ticks and only a few had evidence of circulating antibody. In a separate experiment, no young CD-1 mice, born of infected mothers, had IgM antibody to B. burgdorferi. It would appear that immature mice are not transplacentally infected with spirochetes and must be exposed to infected ticks before becoming infected and infective themselves.