1. Perfusion of the isolated rat liver was accompanied by a rising level of transcobalamin II in the perfusate. There was no change in the concentration of transcobalamin I over the 5 h perfusion period.
2. When [G-14C]leucine was added to the perfusate a 14C-labelled protein appeared in the same position as transcobalamin II on column chromatography.
3. It was concluded that transcobalamin II is synthesized by rat liver.