We found a binding protein for activin and follistatin in serum from female Xenopus laevis and identified it as vitellogenin, which is synthesized in the liver and transported into yolk platelets. Then, we investigated the localization of activin and follistatin proteins in early Xenopus oocytes (stage 6) by electron microscopic immunolabeling with gold colloidal particles. The protein molecules were found to be localized uniformly in oocyte yolk platelets, but not in other cytoplasmic organelles. These findings suggest a novel role of yolk platelets as a reservoir for inductive signals transported by vitellogenin in the differentiation and patterning of cells in Xenopus embryos.