The proteinase present in acetone-dried extracts and in Fruton-type extracts of fresh skin from the rat have been compared and further characterized in the light of Fruton's results.2. The acetone-dried extracts have high pro-teinase activityand low dermo-and amino-peptidase activity with only slight activation effects of manganese, whereas the fresh skin extracts have a high peptidase and low proteinase activity.3. The skin proteinase differs from,previously recognized enzymes.4. Manganese activates the leucine aminopeptidase optimally at 0-002M; cobalt and magnesium also activate.We are indebted to Prof. K. Linderstrom-Lang for his hospitality and kindness in showing his technique; also to the Nuffield Committee and to the Osler Trustees (Oxford)

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