Two-dimensional immunoelectrophoresis and conventional sodium dodecyl sulphate-polyacrylamide gel electrophoresis was performed on various mixtures of purified alpha 1-antitrypsin (alpha 1AT) and leucocyte elastase (LE). The results confirm that alpha 1AT inhibits LE by the formation of enzyme-inhibitor complexes demonstrable by both techniques. The complexes break down with time and are not affected by pH in the presence of excess alpha 1AT. However, the breakdown is more rapid in the presence of excess enzyme only at pH values where LE remains active. The resultant products of the complex breakdown include inactivated LE and alpha 1AT that has undergone limited proteolysis. It is concluded that the presence or absence of alpha 1AT-enzyme complexes as demonstrated by two-dimensional immunoelectrophoresis must be interpreted with caution when studying alpha 1AT function in lung secretions. The absence of such complexes does not mean that previous interaction with enzyme has not occurred, thereby accounting for a reduction in alpha 1AT inhibitory capacity.