Eglin c is an elastase/cathepsin G inhibitor from leech Hirudo medicinalis. The gene for this 70 aminoacid peptide was synthesized chemically, cloned and expressed by E. coli. Here we report biochemical and pharmacological studies. The rate of complex formation between Eglin c and human leukocyte elastase (HLE) or human cathepsin G (H. Cat. G) was determined and compared to those of a number of other proteinase/proteinase-inhibitor interactions (alpha 1 PI and alpha 2M). The association rate constants of Eglin c with the leukocyte enzymes are of the same order of magnitude as those with the naturally occurring inhibitors alpha 1 PI and alpha 2M. The association rate constant of Eglin c (extracted from leech) and Eglin c (biotechnology product) with HLE was found to be identical. The equilibrium constants Ki of the Eglin c/HLE and the Eglin c/H. Cat. G interactions are in the order of 10(-10) M. In an experiment with the hamster emphysema model, 0.5 mg or 2 mg of Eglin c applied intratracheally one hour before an HLE-insult completely protected the animals against emphysema and no signs of toxicity due to Eglin c were observed.