1. A specific method is described for the measurement of angiotensin I converting enzyme activity in plasma with 125I-labelled angiotensin I used as substrate. 2. Converting enzyme activity in plasma from fifteen normal subjects, eleven patients with sarcoidosis, twelve patients with chronic obstructive pulmonary disease and three patients with shock lung was assayed by this technique. 3. Patients with sarcoidosis had increased plasma converting enzyme activity whether or not they were receiving steroid therapy. 4. Patients with chronic obstructive pulmonary disease and shock lung had decreased plasma converting enzyme activity, but extent of conversion did not correlate with the severity of the lung disease. 5. Converting enzyme activity in normal plasma could be completely inhibited by addition of exogenous angiotensin I in 0·5–2·5 × 107 times physiological concentration. Twice as much exogenous angiotensin I was needed to inhibit conversion completely in plasma from patients with sarcoidosis; one tenth as much in chronic obstructive pulmonary disease. These results indicate that plasma has a high capacity for angiotensin I conversion even in patients with pulmonary parenchymal disease. 6. Results suggest that plasma converting enzyme activity may be a reflection of pulmonary conversion and can be altered by pulmonary disease. 7. Measurement of plasma converting enzyme activity may be useful in studies designed to characterize the regulatory role of converting enzyme in the renin—angiotensin system and in cardiovascular homeostasis.