Peripheral polymorphonuclear leucocytes (PMN) from subjects with emphysema or bronchiectasis digested significantly more iodine-125-labelled fibronectin (on average, 250% and 280%, respectively) than did those from control subjects. PMN from patients with bronchiectasis contained significantly more of the serine proteinase elastase than did the control cells, which may have contributed to their greater extracellular proteolysis. PMN from patients with emphysema, but not those with bronchiectasis, showed enhanced chemotaxis (on average 260%) in response to a chemotactic peptide compared with control cells. Thus, PMN from subjects with chronic obstructive lung diseases can digest more extracellular connective tissue protein than PMN from healthy subjects. This behaviour suggests a mechanism for the pathological tissue damage associated with these disorders. Furthermore, the sensitivity to chemotactic factors of PMN from emphysematous patients would contribute to the larger numbers of these cells in their lung tissues, thus increasing further the proteolytic burden in the lungs.