The concentration of leukocyte elastase (ELP) in plasma and serum was determined by an amidolysis method using a specific synthetic substrate for ELP, Suc-Ala-Tyr-Leu-Val-pNA. Results were compared with those using ELISA. ELP levels in plasma from healthy donors were similar to those determined by ELISA; however, the levels in serum were lower than those determined by ELISA. Correlation coefficients of ELP levels in plasma and serum as measured by the two methods were 0.75 (amidolysis) and 0.90 (ELISA). On the other hand, the correlation coefficient between serum ELP by the two methods was 0.83. Half of the ELP levels in plasma from 150 patients and serum from 400 patients were significantly elevated when compared with those from healthy donors, and the ELP elevation determined by amidolytic assay correlated with some variables in blood, namely fibrin(ogen)-degraded products, fibrinogen, GOT, GPT, γ-GTP, LDH and leucine aminopeptidase. Despite the fact that the amidolysis method detects the α₂-macroglobulin-ELP complex while ELISA detects the α₁-antitrypsin-ELP complex, a comparative study showed amidolysis to provide sufficiently sensitive measurement of both plasma and serum ELP.