The reaction between human leukocyte elastase and soybean Bowman-Birk inhibitor has been studied. The inhibition was found to be due to slow tight binding of the inhibitor. The interaction of BBI with HLE was shown to involve two steps: the rapid formation of an initial EI complex, with a K i of 28 nM, followed by a slow equilibrium conversion to a tighter-binding EI * complex with a final K ∗ i of 2.3 nM. At pH 7.5 and 25°C, k on was 3.5×10 4 M −1 s −1 and k off was 1.0×10 −4 s −1. © 1997 Federation of European Biochemical Societies.