1. Oestrogen treatment has previously been shown to induce the formation of large amounts of a serum protein, vitellogenin (xenoprotein), in Xenopus laevis. Vitellogenin was purified from serum by dimethylformamide precipitation and was shown to be homogeneous by a variety of electrophoretic techniques. 2. The molecular weight of vitellogenin was estimated by gel filtration to be about 6×105. The chemical constituents of vitellogenin were determined and lead to the characterization of this protein as a serum calcium-binding glycolipophosphoprotein. 3. The extractable lipid accounted for 12% of vitellogenin. Gas–liquid-chromatographic analysis of the saponified lipid moiety showed the presence of palmitic acid, palmitoleic acid, stearic acid, oleic acid and linoleic acid in the molecular proportions 6.8: 1.5: 1.0: 3.6: 1.4. 4. The carbohydrate moiety consisted of 0.4g of hexose, 0.77g of hexosamine and 0.18g of sialic acid/100g of vitellogenin. 5. The calcium and phosphorus contents were 0.85 and 1.65g/100g of vitellogenin respectively. 6. Serum from oestrogen-treated animals injected with 45CaCl2 contained 9.7 times the radioactivity present in serum from untreated 45CaCl2-injected animals. Of the radioactivity due to 45CaCl2 in the serum of oestrogen-treated animals 72% was non-diffusible on dialysis. Of this activity 65.4% was associated with the vitellogenin band on cellulose acetate electrophoresis.

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