Groups of calves (20 per group) were vaccinated subcutaneously with a single dose of BCG Pasteur (6 x 10(4) or 6 x 10(6) colony forming units) and two months later, 15 calves from each group were challenged intratracheally with virulent Mycobacterium bovis. Vaccination with either dose of BCG induced significant protection against the development of tuberculous lesions compared to non-vaccinated controls. Seven months after BCG vaccination, many of the vaccinated animals which had no lesions and were M. bovis culture-negative at necropsy showed positive reactions for M. bovis in three assays for measuring cellular immune responses (comparative intradermal test, interferon-gamma assay and lymphocyte proliferation assay). This effect was most noticeable in the BCG-vaccinated calves which had been challenged with M. bovis rather than in the non-challenged animals. Antibody responses to M. bovis were very low or absent in the calves during the study. The kinetics of the interferon-gamma response of peripheral blood lymphocytes cultured in vitro with bovine PPD showed that BCG vaccination induced a rapid rise in the response followed by a sharp decline, while infection with virulent M. bovis resulted in an increase in the interferon-gamma response by four weeks after challenge and this response remained high through the study.