Abstract Calf lens nuclear α-crystallin was separated into the three known molecular weight populations on Ultrogel AcA22. Electron microscopy illustrates that the two higher molecular weight classes, which are formed upon aging of the lens, are chain-like oligomers and polymers of the spherical monomeric α-crystallin molecules of 15 nm average diameter and 1·1 × 10 6 average molecular weight which form the lowest molecular weight class. Our combined sedimentation and electron microscopy data make the alleged non-equilibrium state within these populations questionable. From the polypeptide chain composition, determined by isoelectric focusing, it can be inferred that the total amount of C-terminal shortened chains increases progressively from 33 to 53% from monomeric to polymeric α-crystallin. Reassociation of different types of isolated shortened A-chains in various mixtures indicates a tendency for formation of higher molecular weight reaggregates with increasing relative amount of degraded chains. The involvement of degraded chains in polymerization is postulated although it is not likely to be the only factor causing aggregation.