In order to contribute to a better understanding of the role of leukocyte elastase in corneal melting after a severe alkali burn, the in situ appearance and activity of this enzyme in the rabbit cornea was examined. For this reason, a histochemical approach using cryostat sections on membranes and a gel incubation medium with the very sensitive substrate Mu-Ala-Ala-Pro-Val-7-amino-4-trifluoromethylcoumarin (Enzyme Systems Products, Livermore, Calif., USA) was employed. In contrast to the normal rabbit cornea where leukocyte elastase activity was absent, in alkali-burned cornea a high enzyme activity in inflammatory cells (particularly polymorphonuclear leukocytes) was present. The extracellular release of leukocyte elastase into the substantia propria of the corneal stroma was associated with disintegration of the corneal stroma and the appearance of corneal ulcers. These results show that leukocyte elastase is associated with corneal destruction after a severe alkali burn.