The activin/transforming growth factor-beta (TGF-beta) family of peptide growth factors plays a central role in the induction of mesoderm during early Xenopus embryogenesis. Immediate transcriptional responses to mesoderm-inducing signals have been described, but the signal transduction steps leading to these early responses are unknown. We describe here the first pre-transcriptional response to activin/TGF-beta mesoderm inducers in the early embryo. We have identified a cellular factor which binds to a 50 bp portion of the promoter for an activin/TGF-beta early response gene. This factor is activated within 4 min of treatment of embryonic prospective ectoderm with mesoderm-inducing factors, making it the earliest response to these factors described in early embryos. This factor can be activated throughout early cleavage and blastula stages, is activated by mesoderm inducers of the activin/TGF-beta superfamily but not the fibroblast growth factor (FGF) family, and does not appear to require an endogenous FGF signal for activation. Characterization of this factor provides a powerful tool for studying the early steps in the induction of mesoderm by members of the activin/TGF-beta superfamily.