Mycobacterium bovis Bacille Calmette–Guérin (BCG) is the current vaccine used against Mycobacterium tuberculosis (MTB) infection. However, exposure to environmental pathogens, such as Mycobacterium avium, interferes with the immune response induced by BCG vaccination. How M. avium affects the efficiency of BCG is unclear. In this study, BCG-vaccinated mice pre-treated with M. avium-derived lipids (MALs) showed a higher mycobacterial load and increased infiltration of inflammatory cells compared to control mice treated with Escherichia coli-derived lipids (ELs). Unexpectedly, there were no changes in cell proliferation or IFN-γ levels in spleen cells stimulated with protein purified derivatives (PPD) or heat-inactivated BCG in MALs-treated mice. However, pre-treatment with MALs decreased the bactericidal effect as well as the production of TNF-α and nitric oxide (NO) in murine macrophages from BCG-vaccinated mice stimulated with IFN-γ. These results suggest that MAL pre-treatment dampens the immune response against MTB and that this dampening is associated with a decreased response to IFN-γ stimulation in murine macrophages. T-lymphocyte responses, however, were unaffected.