The change of IFN-γ production from whole blood cells stimulated with PPD in mice with increasing age. C57BL/6 mice were subcutaneously vaccinated with BCG (106 CFU) or PBS (unvaccinated). Whole-blood obtained from a) 13-week old, b) 18-week old, c) 24-week old, d) 28-week old, e) 31-week old, f) 40-week old, g) 49-week old, h) 57-week old, i) 83-week old was stimulated with purified protein derivative (PPD, solid column) or PBS (open column) and incubated for 24 h. The concentration of interferon-gamma (IFN-γ) from the culture supernatant was measured by ELISA. Results are shown as mean ± SD from groups of 16 animals. ** p < 0.01,* p < 0.05, N.S.: not significant.