Bovine tuberculosis remains a significant problem in some parts of Great Britain and Ireland largely because of a reservoir of infection in badgers. Little is currently known about the immunopathology of Mycobacterium bovis infection in the badger. Badgers, from 31 social groups, in a study area of the Cotswold escarpment, have been trapped and sampled from 1981 to 1995. Serum antibody responses directed against the 25 kDa antigen (MPB83) of M. bovis have been studied in detail in a selected social group (JM) which has endemic infection. Sequential sera from 44 badgers were studied and results compared with culture from faeces, urine, tracheal aspirates, bite wound swabs and at post mortem. The results indicate that some badgers (about 10%) remain uninfected despite exposure to endemic M. bovis infection within the social group. In culture-positive animals active excretion of organisms is not necessarily concomitant with seropositivity. Conversely, seropositivity is not an indicator that culture positivity is present or imminent. This is particularly true in cubs when a transient seropositivity can occur within the first 6–8 months of life but these animals can remain culture-negative for up to 5 years. Western blotting confirms that at least some of these antibodies, detectable by ELISA in the culture-negative cubs, are directed against the 25 kDa M. bovis antigen. In contrast antibodies detectable in the culture-positive animals do not Western blot prior to a positive culture. Thus, differential reactivity in Western blotting may distinguish between serum antibodies indicative of potentially culture-positive animals and animals which will remain culture-negative.