A method is described for the generation of phenylthiocarbamyl (PTC) amino acids from phenylthiohydantoin (PTH) or anilinothiazolinone (ATZ) amino acids. An aqueous base containing a reducing agent promotes the opening of the PTH or ATZ ring to form a stable PTC amino acid. The new reaction is essentially quantitative for all of the amino acids and may be used instead of the established Edman conversion with an aqueous acid. The PTC amino acids may be analyzed directly or used for generating other analytes. A further method is described for the generation of ATZ amino acids from either the PTH or PTC derivatives. This chemistry involves a Lewis acid-catalyzed dehydration of the PTC. The resulting ATZ may be combined with labels that permit high-sensitivity detection. This reaction is also essentially quantitative for all of the amino acids except aspartic acid, which exhibits a strong preference for the PTH form under these conditions. Taken together, these chemistries allow the preparation of homogeneous PTC or ATZ amino acids from any mixture of amino acid derivatives resulting from the Edman degradation and promise to greatly facilitate the high-sensitivity analysis of protein. (c) 1983 Academic Press, Inc.