Immunoreactive leucocyte elastase was measured in the serum of patients with chronic obstructive bronchitis. No evidence was found to demonstrate the release of this enzyme in the pulmonary circulation. However the average serum concentrations (573 . 0 micrograms/l; SD +/- 261 . 0) were higher (2p less than 0 . 001) in this group of patients than in age matched control subjects (355 . 2 micrograms/l; SD +/- 274 . 8). Further studies confirmed this finding but patients with other active lung diseases had similarly increased leucocyte elastase concentration in the serum. This suggests that a raised serum leucocyte elastase concentration is a feature of active lung diseases and not a feature of obstructive bronchitis alone.