The rate constant for the association between human leukocyte elastase (EC 3.4.21.11) and human bronchial inhibitor has been determined by competition experiments with alpha 1-proteinase inhibitor. This constant (1.1.10(7) M(-1) . s(-1)) is 6-times lower than that for the association of leukocyte elastase and alpha 1-proteinase inhibitor. The latter inhibitor is able to dissociate the leukocyte elastase-bronchial inhibitor complex with a rate constant 1.3.10(-4) s-1. The equilibrium dissociation constant Ki of the complex is 1.2.10(-11) M. The physiopathological significance of these constants is discussed.