Concentrations of leukocyte elastase (ELP) and cathepsin G (CLP) in leukocytes, plasma and serum from healthy human blood were measured using synthetic substrates. The best condition of the extraction of both enzymes from leukocytes was first studied, and the normal values of ELP and CLP in plasma and serum were calculated from the decreased ratio of their activities after the addition of plasma and serum to the pure enzymes. The results obtained were as follows ; 1) The amidolytic activity of

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... LP which obtained by repeated freezing and thawing twenty times in Tris-HC1 buffer (containing 0. 15-2. 0 M NaCI) showed similar value to the activity extracted with 2 M NaClO4. The amidolytic activity of CLP weakened rapidly in the medium, while the extraction of maximum activity was obtained by repeated freezing and thawing in the above mentioned buffer. 2) The amidolytic activities of ELP and CLP were decreased to 4 % and 45%, respectively, by the addition of normal plasma or serum. 3) Normal