Summary. Homogenates of normal human muscle showed cathepsin activity over a wide pH range, with peaks at pH 3.9 and 6.6. Of various proteins tested, denatured myoglobin was the best substrate at pH 4; some proteins depressed activity. In human dystrophic muscle (Duchenne) there was increased cathepsin activity (based on non-collagen protein) in the acid, neutral and alkaline range.