solution after removal of the leukocytes with a compound of general formula: Y--X--C in which Y is an amino acid or a peptide containing 2 or 3 amino acids, X is arginine or lysine and C is a chromogenic residue, in the presence of a buffer effective at pH 6.3 to 8.0, said protease liberating the chromogenic residue, and thereafter measuring the chromogenic residue. 2.