(A) IFN- induces tyrosine phosphorylation of STAT1 and STAT4 in human T lymphocytes. T cells were treated with IFN- or IL-12 for 15 minutes, and T-cell lysates were prepared and immunoprecipitated with anti-STAT1, anti-STAT3, or anti-STAT4 antibodies. Proteins were separated on 10% SDS-PAGE, transferred to membranes, and immunoblotted with antiphosphotyrosine antibody. The membranes were stripped and reblotted with anti-STAT antibodies. (B) IFN- induces tyrosine phosphorylation of STAT5a and STAT5b. T cells were treated with IFN- or IL-2 for 15 minutes and T-cell lysates were prepared. The lysates were immunoprecipitated with anti-STAT5a or anti-STAT5b antibodies, followed by immunoblotting with antiphosphotyrosine antibody. Membranes were then stripped and reblotted with anti-STAT antibodies, as indicated.