1. Peripheral airspaces were selectively lavaged using balloon-tipped catheters of 2 mm diameter. 2. Molar anti-leucoprotease/alpha 1-proteinase inhibitor ratios as determined in these lavage fluids by enzyme immunoassays ranged from 0.02 to 0.14 (mean 0.08). 3. These data indicate that peripheral airspaces contain minor amounts of anti-leucoprotease relative to alpha 1-proteinase inhibitor. This observation does not necessarily mean that anti-leucoprotease is of minor importance in protecting the lung interstitium.