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 deamidationin 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.