This study was conducted to provide a description of calpain proteolytic enzyme (EC 3.4.22.17) in normal rat cornea and to document immunohistochemical changes in calpain distribution during maturation. Corneal soluble proteins were fractionated by diethylaminoethyl chromatography on high-performance liquid chromatography. Fractions were analyzed for calpain by enzyme-linked immunosorbent assay, immunoblotting, and caseinolytic enzyme activity with fluorescein isothiocyanate-labeled casein. Calpain II from the soluble fraction of 2-week-old and 3-month-old rat corneas eluted at a similar NaCl concentration (220-240 mmol/l) as calpain II from other tissues, was inhibited by E64, contained an 80-kilodalton subunit in immunoblots, and was present at specific activity of 473 units per gram of protein in 3-month-old rats and 801 units per gram of protein in 2-week-old rats. Calpain antigen also was present in the ethylenediaminetetraacetic acid and EGTA washed insoluble fraction of cornea. Calpain was found (by immunohistochemical analysis) in all layers of the epithelium but not in the stroma. Enzyme-linked immunsorbent assay, immunoblots, and immunohistochemical analysis showed that calpain in the whole cornea did not change with corneal maturation. It was hypothesized that calpain in the cornea may be involved in the turnover of epithelial cells during normal maturation.