1. A study has been made of the copper- and zinc-binding proteins in the livers from sheep foetuses of 80-142 d gestational age. 2. Metallothionein was found to constitute the major Zn-binding component in the cytosol at all times and to be identical to Zn-thionein from adult sheep liver. 3. Zn also occurred in two fractions, not normally found in sheep liver, with approximate molecular weights of 28000 and 47000. The relative proportions of these were age-dependent. 4. Between 15 and 35% of the hepatic Cu, corresponding to most of the Cu in the cytosol, also occurred in the metallothionein-containing fraction.