Slices of liver from laying hens incorporated Na(2) (14)CO(3) and NaH(2) (32)PO(4) into phosvitin. Slices of liver from immature birds did not do so to any appreciable extent. The (32)P was incorporated into O-phosphorylserine in the phosvitin molecule. 2. Kidney, spleen, muscle, large and small intestine, ovary and oviduct from laying birds did not incorporate Na(2) (14)CO(3) into phosvitin. 3. Slices of liver from laying hens carried out a net synthesis of phosphoprotein under the standard conditions of incubation. Slices from the livers of immature pullets did not do so. 4. Liver from the laying hen incorporated [2-(14)C]glycine, [3-(14)C]serine and [2-(14)C]glutamic acid into phosvitin. Part of the glycine was shown to be present as serine in the final product. 5. Slices of liver from immature birds treated with oestradiol synthesized phosvitin from [2-(14)C]glycine, but the addition of oestrogens in vitro to slices from untreated immature birds did not promote synthesis during a 3 hr. incubation period.
