Abnormal activation of the protease calpain in the lens may be a cause of cataracts. Cataracts were induced in 10-day-old rats by a single overdose of sodium selenite. The water-insoluble protein from the opaque lens nucleus was separated by two-dimensional electrophoresis, electroblotted onto membranes, and the NH2-terminal sequence of partially degraded beta-crystallin polypeptides determined. Selenite cataractous lenses contained four major structural proteins, beta B1, beta B3, beta A3/A1, and beta A4 crystallins, missing from 5 to 49 amino acids from their NH2 termini. Incubation of intact beta-crystallins with calpain II in vitro produced identical cleavage sites. This provided further evidence for the role of calpain in the production of light scattering insoluble protein in cataractous lenses and also suggested that a similar process may lead to lens protein insolubilization during aging.