BCG protects both young and aged mice against M. tuberculosis infection. Young (a) and aged (b) mice were immunized with 1 × 106 CFU of BCG (solid column) or PBS (open column). Eight weeks after immunization, the mice were intravenously challenged with 1 × 105 CFU of M. tuberculosis strain H37Rv. Four weeks after challenge, the bacterial numbers in the lung and spleen were determined by colony assay. Data represent mean ± SD of three mice. *, p < 0.05, compared with PBS-inoculated control group (student's t-test).