The aim of this study was to compare protection by bacille Calmette-Guérin (BCG) vaccination against aerosol challenge with either Mycobacterium bovis or Mycobacterium tuberculosis in guinea pigs. Animals were challenged 5 weeks after vaccination with 10 or 100 lung lesion-forming units (lfu) of M. bovis or M. tuberculosis. Four weeks after challenge, numbers of lung lesions and counts of viable mycobacteria in spleens were high in saline-immunized animals. In contrast, BCG vaccination resulted in fewer lung lesions; after challenge with 10 and 100 lfu, the reduction was greater in animals infected with M. bovis (mean number of lesions, 1.17 and 31.2, respectively; P<.02) than in those infected with M. tuberculosis (mean number of lesions, 16.2 and 75.8, respectively). No mycobacteria were recovered from spleens of BCG-vaccinated animals after challenge with 10 lfu of M. bovis, whereas 4 of 6 animals had detectable spleen mycobacterial counts after challenge with M. tuberculosis. Collectively, the results suggest that BCG vaccination may confer greater protection against challenge with M. bovis than challenge with M. tuberculosis.