There are several lines of evidence that activin is a crucial molecule for mesoderm induction in early Xenopus embryos. However, it is not known what kind and what amounts of activin proteins exist in cleavage stage embryos. Three isoforms of activins, A, AB, and B, were demonstrated to be present at least in part as a complex with abundant follistatin, an activin-binding protein, in early Xenopus embryos (stage 1-5). These results suggest that activin stored in eggs has an important role for mesoderm induction during early Xenopus embryogenesis and that follistatin can modulate the function of activin in signaling developmental changes.