IFN-γ augments while IL-4 inhibits the IL-12–induced tyrosine phosphorylation of STAT1α and STAT1β. PBMC were cultured for 72 hours with PHA, PHA + 1,000 U/mL IFN-γ (A), or PHA + 20 ng/mL IL-4 (B); rested overnight in fresh medium containing 2.5% fetal calf serum; and then stimulated for 20 minutes with either medium alone (lanes 1 and 4), 100 pmol/L IL-12 (lanes 2 and 5), or 100 pmol/L IL-2 (lanes 3 and 6). Western blots were performed with antibodies to phospho-STAT1 (A, upper panel, and B) or STAT1 (A, lower panel). Results are representative of five separate experiments.