IFN-gamma has been shown to either up- or down-regulate the expression of specific TGF-beta1-induced target genes. We investigated the effect of IFN-gamma on TGF-beta1-induced IgA isotype expression. We found that IFN-gamma inhibited not only TGF-beta1-induced germ-line (GL) alpha transcription, but also IgA secretion by TGF-beta1-stimulated murine B cells. Overexpression of Stat1 diminished TGF-beta1-induced, Smad3/4-and Runx3-mediated GL alpha promoter activity. Overexpression of p300 also increased the promoter activity, while its effect was abrogated by co-transfected Stat1. Stat1 interfered with the Smad3:p300 interaction, likely due to a stronger Stat1:p300 binding affinity. These results indicate that Stat1 can inhibit GL alpha transcription through binding to p300. Further, overexpression of SOCS1, a JAK inhibitor, diminished the antagonistic effect of IFN-gamma on TGF-beta1-induced GL alpha transcription and IgA secretion. These results indicate that JAK/Stat1-mediated IFN-gamma signaling antagonizes TGF-beta1-induced GL alpha transcription, mainly through deprivation of p300 from Smad3, resulting in decreased IgA synthesis.