The Major Intrinsic Polypeptide (MP26) of lens membranes contains an -asn-gly- sequence, which has been shown in other proteins to be particularly susceptible to spontaneous deamidation. To determine if the asparagine residue of this sequence undergoes age-dependent deamidation in vivo, antiserum to a synthetic peptide containing the sequence was used to monitor purification of a tryptic peptide containing this sequence from fetal versus mature bovine lenses. The peptide from fetal lenses contained the -asn-gly- sequence, while the peptide from mature lenses contained an -asp-gly- sequence, demonstrating that age-dependent deamidation of this asparagine residue was occurring in the lens.