Vg1 is a maternal mRNA localized in the vegetal cortex of Xenopus laevis oocytes, that encodes a protein homologous to the mammalian growth factor TGF-beta. Using a polyclonal antibody to a T7-Vg1 fusion protein, we have identified the native protein. We find that a single protein of Mr 40 kd is immunoprecipitated following in vitro translation of oocyte poly(A)+ RNA, whilst two proteins of Mr 45 and 43.5 kd are immunoprecipitated from oocyte and embryo extracts. Synthesis of at least the 40 kd, in vitro, and 45 kd, in vivo, proteins is specifically inhibited following treatment of the respective systems with antisense Vg1 (but not histone H4) oligodeoxynucleotides. Tunicamycin treatment reveals the in vivo proteins to be glycosylated versions of a 40 kd protein, modified by the addition of either two or three N-linked oligosaccharide side chains. Both proteins are sensitive to digestion by the enzyme endoglycosidase-H, and are segregated within a membrane fraction from which they can be released by high pH treatment. Their synthesis is first detectable in stage IV oocytes and continues throughout early embryogenesis until the late gastrula. During embryogenesis the relative proportions of the two proteins change, the 45 kd protein being predominant in early embryogenesis and the 43.5 kd protein in late embryogenesis. Synthesis only occurs in the vegetal hemisphere at all stages; however, in the large oocyte diffusion of both proteins into the animal hemisphere occurs.