In infected IFN-γ −/− mice, the T-cell response to BCG is profoundly shifted to the Th2 phenotype. Splenocytes from infected IFN-γ +/+ mice (left side of each panel) or IFN-γ −/− mice (right side of each panel) were stimulated in vitro with media alone (open bars) or PPD (filled bars). Cytokine levels were measured by ELISA after 72 hours stimulation and expressed in pg/mL. In the experiment shown, mice on the C57BL/6 background were used. Similar results were obtained using IFN-γ −/− on the Balb/c background.