Polypeptides can be prepared by stirring an amino acid solution in the presence of diphenylphosphoryl azide (DPPA) and an adequate tertiary amine. Generally, 1,3 mol of DPPA and 2,3 mol of triethylamine per mol of amino acid gave the best result. Polymerization of various amino acids (Gly, Ala, Leu, Phe, Met, Glu(OBzl), Glu(OMe), Cys(Bzl), Ser(Bzl), His(Bzl), Lys(Z), Arg(Tos), Arg(N02), Arg(Mts), etc.; cf. Tab. 2) in dimethyl sulfoxide, N,N-dimethylformamide, 1,4-dioxane, pyridine, etc. gave the corresponding polypeptides with molecular weights as estimated from their intrinsic viscosities between 3 000 and 16000, in reasonable yields. The purity of the resulting polypeptides was confirmed by IR, elemental analysis and optical rotation measurements of their acid total hydrolysates.