Activin, a peptide growth factor, is a member of the transforming growth factor-β (TGF-β) superfamily. It was originally isolated from follicle fluid as a gonadal hormone that stimulates follicle-stimulating hormone (FSH) secretion and is identical to EDF, the erythroid differentiation factor (which stimulates erythroleukemia cells to differentiate into hemoglobin-producing cells) (1 ). It also appears to be related, if not identical, to the so-called “vegetalizing factor”described originally by Tiedemann and colleagues, which can induce amphibian embryonic tissue rudiments to display various differentiation patterns (reviewed in ref. 2 ). In addition, it appears to be identical to the XTC factor isolated by Smith et al. (3 ) from transformed Xenopus fibroblasts. Thus, the identification of activin as a potential morphogen in amphibian embryos (4 ) solved several mysteries surrounding the puzzle regarding the molecular nature of various hitherto ill-characterized “inducing substances.“