Human alpha-crystallins were separated from fetal, young, senile nondiabetic and diabetic lenses. The effects of aging and diabetes mellitus were studied by fluorescence measurements, including emission maximum, quantum yield and polarization, using both intrinsic probes (tryptophan and non-tryptophan) and extrinsic probes [4-(N-iodoacetoxy)N-methylamino-7-nitrobenz-2-oxa-1,3-diazole (IANBD) and 6-(p-toluidinyl)naphthalene-2-sulfonate (TNS)]. Results indicate that diabetic effects (glycation and aggregation) give fluorescence change to a far greater extent than that of aging. This was demonstrated by a large decrease in tryptophan quantum yield and an increase in non-tryptophan quantum yield, and also by a decrease in polarization of non-tryptophan. The sulfhydryl (SH)-specific probe IANBD shows a blue-shift in emission maximum, a decrease in intensity and an increase in polarization. The hydrophobic probe TNS shows a decrease in both intensity and polarization. These results suggest that tryptophan oxidation, mixed disulfide formation and glycation, as well as other undetected post-translational modifications have partially unfolded the proteins, making the protein structure less rigid. The possible effect of this unfolding process is that proteins become more susceptible to aggregation.