Tumor necrosis factor-⍺ (TNF-⍺) plays critical and opposing roles in the pathogenesis of tuberculosis (TB). We examined the effects of Mycobacterium bovis BCG vaccination on TNF-⍺ production in three distinct guinea pig leukocyte populations before and after pulmonary infection with M. tuberculosis H37Rv. Following BCG vaccination alone, and following challenge, bronchoalveolar lavage cells (BALC), resident peritoneal cells (PC), and splenocytes (SPC) were stimulated with purified protein derivative (PPD). Before virulent challenge, BCG vaccination clearly enhanced the ability of BALC, PC and SPC to produce TNF-⍺ in response to PPD stimulation ex vivo. Following challenge, the TNF-⍺ production of all three leukocyte populations from BCG-vaccinated animals remained relatively constant at pre-challenged levels. In sharp contrast, 5 weeks post-challenge, all three leukocyte populations from unvaccinated animals produced very high amounts of TNF-⍺ in response to PPD. Three weeks post-challenge, SPC from one of the unvaccinated animals produced higher levels of TNF-⍺ but the others produced lower levels of TNF-⍺ than BCG-vaccinated animals. As expected, BCG vaccination reduced the levels of virulent mycobacteria in both the lungs and spleens. Thus, BCG vaccination allows guinea pigs to modulate TNF-⍺ levels in conjunction with a reduction in bacillary loads in their tissues.