Abstract 1. Preparation of four copoly-(l-glutamic acid, Dl-serine), the molar ratios of the two amino acid residues were 4:1, 3:2, 2:3 and 1:4, by debenzylation of the corresponding four copoly-(γ-benzyl l-glutamate, O-benzyl Dl-serine), was attempted. 2. The inter- and/or intra-molecular ester-linkages, formed during the debenzylation reaction between hydroxyl group of serine residue and carboxyl group of glutamic acid residue, was broken by treating with aqueous triethylamine without accompanying racemisation or hydrolysis of the peptide bond. 3. Copoly-(l-glutamic acid, Dl-serine) became freely soluble in water when the content of serine residue was more than about 50 mole per cent. 4. Copoly-1:1:2-(l-glutamic acid, l-lysine, Dl-serine), prepared by debenzylation from the parent copolymer, was soluble in water over a whole pH range, and the pH dependence of viscosity and of optical rotation was measured on its solutions. It was found that these properties are very similar to those of natural albumins.