Signal transducer and activator of transcription (Stat) 6 is vital to interleukin (IL)-4 and IL-13 responses and the generation of Th2 immunity. We investigated the cellular location of phosphorylated Stat6 and Stat6 DNA binding activity in A201.1 murine B cells and primary splenocytes. Phosphorylated Stat6 was present in cytoplasmic and nuclear extracts from IL-4-treated cells. Confocal microscopy confirmed the presence of phosphorylated Stat6 in the cytoplasm of IL-4-treated cells. In contrast, Stat6 DNA binding activity was present in nuclear extracts, but not in cytoplasmic extracts. Thus, cytoplasmic extracts from IL-4-stimulated cells were devoid of Stat6 DNA binding activity despite the presence of phosphorylated Stat6. Addition of cytoplasmic extracts to nuclear extracts did not inhibit Stat6 DNA binding present in the nuclear extracts. Detergent treatment restored Stat6 DNA binding activity in cytoplasmic extracts of IL-4-stimulated cells. Thus, DNA binding activity of cytoplasmic phosphorylated Stat6 is masked by a factor dissociable by detergent treatment.