An improved procedure for high-sensitivity microsequencing is described. The method employs (i) aminoethyl aminopropyl glass beads for in situ purification of quadrol by absorbing α-amino group reactive impurities commonly found even in highly purified quadrol and (ii) an Altex 5-μm Ultrasphere ODS column and highly purified methanol which is available commercially to identify phenylthiohydantoin derivatives of amino acids by high-pressure liquid chromatography. The efficiency of the method has been demonstrated by the results of sequence analyses of peptides obtained by acid and cyanogen bromide digestions of viral proteins. A significant increase in the initial and repetitive yields has been consistently observed by this procedure. © 1981.