During the manufacture of a proprietary peptide drug substance a new impurity appeared unexpectedly. Investigation of its chemical structure established the impurity as a beta-Ala insertion mutant of the mother peptide. The source of the beta-Ala was identified as contamination of the Fmoc-Ala-OH raw material with Fmoc-beta-Ala-Ala-OH. Further studies also demonstrated the presence of beta-Ala in other Fmoc-amino acids, particularly in Fmoc-Arg(Pbf)-OH. In this case, it was due to the presence of both Fmoc-beta-Ala-OH and Fmoc-beta-Ala-Arg(Pbf)-OH. It is concluded that beta-Ala contamination of Fmoc-amino acid derivatives is a general and hitherto unrecognized problem to suppliers of Fmoc-amino acid derivatives. The beta-Ala is often present as Fmoc-beta-Ala-OH and/or as a dipeptide, Fmoc-beta-Ala-amino acid-OH. In collaboration with the suppliers, new specifications were introduced, recognizing the presence of beta-Ala-related impurities in the raw materials and limiting them to acceptable levels. The implementation of these measures has essentially eliminated beta-Ala contamination as a problem in the manufacture of the drug substance.