Cathepsin-B activity was determined fluorimetrically in the otosclerotic stapes footplate, the stapes superstructure, normal temporal cortical bone, and os frontale osteoma. Measurements with a synthetic substrate made determinations in individual samples possible. The cathepsin-B activity in the otosclerotic stapes footplate was one order of magnitude higher than that of the superstructure, which was not affected by the disease. The cortical bone and the superstructure displayed similar activities, as did os frontale osteoma and otosclerosis. The high lysosomal proteinase activity appears to be closely connected to the otosclerotic bone resorption process.