DURING the determination by Edman degradation of the amino-acid sequence of small peptides and the C-terminal region of longer peptides, extensive losses of material can occur when the 2-methylamino-5-thiazolinone obtained from the N-terminal amino-acid is extracted from the remaining peptide. This problem is encountered in degradations in which the thiohydantoin is identified directly as well as in the subtractive Edman degradation^2,3. In the latter procedure the amino-acids in the peptide are determined before and after removal of the N-terminal residue; the identity of the N-terminal amino-acid is established by difference. Because a separate peptide hydrolysis step and amino-acid analysis is required for each amino-acid in the sequence, the subtractive procedure is also relatively time consuming.