Species variability of the lens protein zeta-crystallin was correlated with those of alcohol dehydrogenases of classes I and III and sorbitol dehydrogenase in the same protein family. The extent of overall variability, nature of residues conserved, and patterns of segment variability, all fall within the limits typical of the 'variable' group of medium-chain alcohol dehydrogenases. This shows that zeta-crystallin is subject to restrictions similar to those of classical liver alcohol dehydrogenase and therefore derived from a metabolically active enzyme like other enzyme crystallins. Special residues at the active site, however, differ substantially, including an apparent lack of a zinc-binding site. This is compatible with altered functional properties and makes the spread within this medium-chain dehydrogenase family resemble the wide spread within the short-chain dehydrogenases. Schematic plotting is useful for illustrating the differences between 'variable' and 'constant' enzymes.