Poly-(α-benzyl L-aspartate) has been prepared by the polymerisation of α-benzyl β-N-succinimidyl L-aspartate and converted into poly-(β-L-aspartic acid) by hydrogenolysis; treatment of the poly(benzyl ester) with hydrogen bromide in acetic acid leads to a different polymer containing a substantial proportion of aspartimide residues. The analogous route cannot be used for the preparation of poly-(α-benzyl L-glutamate), which was best prepared by the polymerisation of αα′-dibenzyl γ-N-succinimidyl γ-L-glutamyl-L-glutamate; poly-(γ-L-glutamic acid) was obtained from poly-(α-benzyl L-glutamate) by the action of hydrogen bromide in acetic acid. The poly-acids had molecular weights of about 11,000 and no racemisation accompanied the polymerisations.