Determination of a protein's amino acid sequence by the generation of a series of fragments from the original protein which represent sequences of single amino acid removals. The series is generated by stepwise degradation effected by Edman degradation using phenylisothiocyanate and a small amount of a termination reagent, phenylisocyanate. Cycles of reaction are performed without isolation of intermediates, and the products are characterised by mass spectrometry.