Antiserum against the N-terminal peptide of bovine alpha-A crystallin has been used to monitor purification of two different sero-positive peptides (i.e. Tla and Tlb) from a tryptic digest of bovine lens proteins. Both these peptides have similar amino acid compositions, but peptide Tlb has a molecular weight 16 atomic mass units larger than Tla, suggesting posttranslational modification. Analysis of ionization fragments of the Tlb peptide by mass spectrometry demonstrates that this difference in molecular weight is due to thein vivooxidation of the N-terminal met residue of the alpha-A crystallin molecule.