Phosphorylation of STAT5a and STAT5b. Indicated 32Dcl3 cell lines were stimulated with diluent (D), 200 pmol/L mGM-CSF (M), or 400 pmol/L hGM-CSF (H), and lysed. Lysates were immunoprecipitated with anti-STAT5a (A) or anti-STAT5b (B) antibody or 2 mg mouse IgG or normal rabbit serum (*) as a negative control. Immunoprecipitates were subjected to SDS-PAGE (7.5% gel) and transferred to Hybond-ECL filters. Filters were probed with 4G10 antiphosphotyrosine antibody, stripped, and reprobed with anti-STAT5a or anti-STAT5b antibodies.