Sites of synthesis of plasma proteins in the foetal rat.

1. The foetal rat of 16 or more days incorporates (14)C-labelled amino acids into all the demonstrable plasma protein fractions in vivo. 2. Slices of foetal rat liver incubated in vitro incorporate (14)C-labelled amino acids into the main plasma protein fractions, including the foetal-specific ;post-albumin'. 3. Slices of placenta are unable to incorporate (14)C-labelled amino acids into plasma proteins in vitro. 4. Liver slices from maternal rats incubated in vitro incorporate (14)C-labelled amino acids into plasma proteins. The presence of post-albumin cannot be demonstrated after incubation. 5. Liver slices from foetal rats, but not from adult rats, contain demonstrable amounts of haemoglobin into which (14)C-labelled amino acids are incorporated.