ing after removal of the leukocytes with a compound of the general formula: Y--X--C wherein Y is an amino acid or a peptide containing 2 or 3 amino acids and includes the group Gly-Pro; C is a chromogenic residue and represent a p-nitroaniline group; and X is arginine or lysine whereby the chromogenic residue C is split-off, and thereafter, testing for the split-off chromogenic residue. 15.