The electrophoretic mobility of purified alpha 1-proteinase inhibitor was compared with that of carbamoylated transferrin. The results ranged from 64.0 to 68.9% of the distance moved by the transferrin and was increased by cigarette smoke solution (range 70.4% to 75.0% of carbamoylated transferrin). The addition of leucocyte elastase produced a change in electrophoretic mobility only in the presence of excess enzyme when mobility fell (58.0 to 62.0%) and was associated with complete and not partial loss of inhibitory activity. No further change was seen over 24 h. Studies on sputum showed a wide range of mobility from 68.0 to 45.0% but only those with a mobility greater than 64.0% retained any inhibitory capacity against porcine pancreatic elastase. However, several samples had a mobility lower than that produced by proteolysis with leucocyte elastase and some showed continuing reduction with time. It is suggested that this is due to proteolysis by more than one enzyme.