Previous studies have demonstrated that a major site of in vivo phosphorylation of alpha-A crystallin from human lens is serine-122. To determine the relative degree of this phosphorylation in alpha-A crystallin from human lenses of different age, alpha-A crystallin was purified from total lens proteins, followed by sequential digestion with lys-C and asp-N endoproteases. Mass spectral analysis of the asp-N peptide fragments that contained serine-122 demonstrated undetectable levels of phosphorylation from infant human lenses (41 days, 2 months and 4 months of age). Identical analysis of alpha-A crystallin from older lenses (12, 15, 40 and 73 years of age) indicated significant phosphorylation of serine-122, demonstrating that phosphorylation of the serine-122 residue of alpha-A crystallin does not occur during the aging process, but is rather a developmentally regulated event in the human lens.