Transforming growth factor (TGF)-beta signals through a heteromeric complex of serine/threonine kinase receptors. The type I receptor phosphorylates and activates the receptor-regulated Smads (R-Smads), Smad2 and Smad3, which form hetero-oligomeric complexes with the co-Smad, Smad4, and translocate to the nucleus. Smad3 and Smad4 can bind directly to consensus DNA-binding elements in the promoters of target genes, whereas Smad2/Smad4 complexes are targeted to DNA by interacting with sequence-specific DNA-binding transcription factors that contain a well-defined Smad interaction motif (SIM). The exact stoichiometry of Smad homo- and hetero-oligomers both before and after ligand stimulation is controversial. Here we determine the stoichiometry of TGF-beta-induced Smad-transcription factor complexes on DNA. We show that complexes of Smad2/Smad4 with the transcription factors Fast-1 or Fast-3 contain one Fast, two Smad2s, and one Smad4. In contrast, Smad3/Smad4 complexes that bind the Smad-binding element from the c-jun promoter, are heterodimers. Furthermore, these Smad3/Smad4 complexes contain at least two additional components essential for complex formation, one of which contains a SIM. Our data suggest that the R-Smads can form heterodimers or heterotrimers with Smad4, and we propose that the exact stoichiometries of active Smad complexes on DNA may be determined by the transcription factors with which they associate.