The protective activity against TB in young and middle-aged mice. BCG-vaccinated or unvaccinated mice, young (30-week old) (a, b) or middle-aged (89-week old) (c, d), were intravenously challenged with 1 × 105 CFU of M. tuberculosis strain H37Rv. Middle-aged mice (c, d) were non-boosted (open column), 3-times boosted by Oligo-B (solid column). Four weeks after challenge, the bacterial numbers in the spleen (a, c) and lung (b, d) were determined by colony assay. Data represent mean ± SD from 8-10 mice. The results of Oligo-B-boosted mice were obtained from 4 mice. ** p < 0.01, * p < 0.05, N.S.: not significant.