Borrelia burgdorferi expresses a conserved, species-specific 39-kDa protein (P39) that can stimulate antibodies during human infection. To confirm that anti-P39 antibodies are produced consistently in animals exposed to infectious spirochetes, white-footed mice, Peromyscus leucopus, and laboratory white mice, Mus musculus (strain BALB/c), were experimentally inoculated with either infectious or noninfectious B. burgdorferi and the antibody response to P39 was determined by immunoblot at 21 days postinoculation. All mice inoculated with approximately 10(7) infectious B. burgdorferi produced anti-P39 antibodies and were cultured positive for this spirochete. Mice inoculated with similar numbers of inactivated or viable noninfectious B. burgdorferi still producing P39 did not induce anti-P39 antibodies. By contrast, putative antiflagellin antibodies were detected in less than 18% of the infected animals, which supports the notion that antibody reactive with flagellin may not be reliable as a marker for B. burgdorferi exposure as was originally thought. Mice infected with B. burgdorferi following exposure to ticks (Ixodes dammini) produced anti-P39 antibodies no later than 7 days postinfection, indicating that P39 is an effective immunogen in natural infections. Notably, anti-P39 antibodies were the predominant B. burgdorferi reactive antibodies detected early in the infection. Our results indicate that anti-P39 antibodies are produced in response to an active infection and are therefore reliable markers for infection in experimentally and naturally inoculated animals.