TGF-beta1 directs class switching to IgA by splenic B cells and by the surface IgM+ B cell line, I.29mu, by inducing germline (GL) Ig alpha transcripts. The promoter segment between -130 and +46, relative to the first initiation site for mouse GL alpha transcripts, is sufficient for expression and TGF-beta1 inducibility of a reporter gene in B cell lines. Within this segment resides a TGF-beta1-responsive element (TbetaRE) that is required for induction of the promoter by TGF-beta1 and, when multimerized, is sufficient to transfer TGF-beta1 inducibility to another promoter. In this report we show that a TGF-beta1-inducible complex binds the TbetaRE and contains the transcription factor core-binding factor (CBF; also known as acute myeloid leukemia, AML). Although all three CBF alpha family members activate the GL alpha promoter, only CBF alpha3 (AML-2) is induced by TGF-beta1 in splenic B and I.29mu cells. The TbetaRE contains two CBF binding sites. Mutation of both sites reduces but does not eliminate induction of the GL alpha promoter by TGF-beta1 or by overexpression of CBF, possibly due to the presence of an additional CBF site in the promoter. In addition, the TbetaRE contains two copies of another sequence motif. Mutation of these motifs eliminates TGF-beta1 induction of the GL alpha promoter. Together the data indicate that TGF-beta1 induction of the alpha promoter involves induction of CBF alpha3, which binds to the TbetaRE of the promoter along with one or more proteins.