A protein (vegetalizing factor) which induces amphibian gastrula ectoderm to tissues which in normal development are derived from endoderm and mesoderm has been isolated from chicken embryo trunks by a combination of size exclusion and reversed phase HPLC. An amount of 0.5 ng factor per gastrula evokes inductions in 80-100% of the cases. The protein (apparent Mr approximately 13 000) is split by NaBH4 to polypeptide chains of about half the size under conditions in which disulfide bridges are reduced. The biological activity is lost.