Signal transducer and activator of transcription 5b (STAT5b) is constitutively activated in many human tumors. Activity of STAT5b requires binding of its Src homology 2 (SH2) domain to certain phosphotyrosine-containing sequences. We have developed a highthroughput assay based on fluorescence polarization that allows screening of chemical libraries for compounds that inhibit STAT5b by interfering with the function of its SH2 domain. The assay, which is based on binding between a fluorescein-labeled phosphotyrosine peptide derived from the erythropoietin receptor to the STAT5b SH2 domain, is stable with regard to dimethyl sulfoxide concentration and time and has a Z0 value of 0.66 ± 0.11 in a 384-well format.