Transcription of the germline Cγ1 and Cε Ig genes is believed to be a necessary prerequisite for isotype switching to IgG1 and IgE, respectively. IL-4 stimulation and ligation of CD40 can each independently induce low level germline γ1 and ε transcription in murine B cells. Together these signals act synergistically to promote high level germline transcription and are normally required for T-dependent isotype switching to IgG1 and IgE. The STAT6 transcription factor has been suggested to play a critical role in IL-4-induced activation of germline Cγ1 and Cε genes. To directly assess the role of STAT6 in IL-4R-and CD40-mediated germline transcription and switching, we have analyzed these events in splenic B cells from STAT6-deficient mice. Our results demonstrate that IL-4 does not induce detectable levels of germline γ1 or ε transcripts in STAT6-deficient B cells. Germline transcript expression induced by CD40 stimulation alone is unaffected, but synergism between CD40-and IL-4R-mediated signals is completely ablated. Switch recombination to Sγ1, as measured by digestion-circularization PCR, is dramatically reduced in STAT6-deficient B cells stimulated with CD40 ligand plus IL-4. Similarly, germline γ1 transcript expression and switch recombination to Sγ1 are also impaired in STAT6-deficient B cells stimulated with IL-4, IL-5, and anti-IgD Abs conjugated to dextran, a model for T-independent type II responses. These results directly demonstrate a critical role for STAT6 in the IL-4-mediated activation of germline Ig gene transcription and switch recombination in nontransformed B cells.