Taxon-specific crystallins are proteins present in high abundance in the lens of phylogenetically restricted groups of animals. Recently it has been found that these proteins are actually enzymes which the lens has apparently adopted to serve as structural proteins. Most of these proteins have been shown to be identical to, or related to, oxidoreductases. In guinea pig lens, which contains zeta-crystallin, a protein with an NADPH-dependent oxidoreductase activity, the levels of both NADPH and NADP+ are extremely high and correlate with the concentration of zeta-crystallin. We report here nucleotide assays on lenses from vertebrates containing other enzyme/crystallins. In each case where the enzyme/crystallin is a pyridine nucleotide-binding protein the level of that particular nucleotide is extremely high in the lens. The presence of an enzyme/crystallin does not affect the lenticular concentrations of those nucleotides which are not specifically bound. The possibility that nucleotide binding may be a factor in the selection of some enzymes to serve as enzyme/crystallins is considered.