A novel method for polypeptide synthesis, in which partially protected peptide thioesters are used as building blocks, has been developed. Partially protected peptide thioesters are easily prepared by solid-phase methodology. The thioester moiety is converted to an active ester in the presence of a silver compound such as AgNO 3 or AgCl and an active ester component such as 1-hydroxybenzotriazole or 3,4-dihydro-3-hydro-4-oxo-1,2,3-benzotriazine. Segment condensation can be accomplished using partially protected peptide segments. The consecutive condensation of the partially protected peptide segments is realized by the selective removal of the 9-flourenylmethoxycarbonyl group, for terminal amino protection, after segment condensation has been achieved. In this method, large peptide segments can easily be used. Thus, the products obtained by the thioester method can be separated from by-products by reverse phase high performance liquid chromatography, even when no purification process was performed during the prior segment condensation procedures. This indicates that proteins that have no specific features such as enzymatic or biological activities can be obtained after isolation, solely based on their chromatographic profiles. Thus, the thioester method will provide a new basis for protein studies including phosphorylated and glycosylated polypeptides.