Surface activity of atelectatic lung extracts was studied with a surface film balance. All 16 atelectatic specimens demonstrated decreased surface activity regardless of etiology. Of those 12 not associated with pneumonia, contiguous lung tissue was normally surface active. Only atelectatic portions, no matter how tiny, were inactive. To clarify factors involved with the 4 atelectatic specimens associated with pneumonia, 23 specimens of pneumonia were studied. All produced inactive extracts. Eleven of 19 contiguous sections studied were also abnormal. Into this group fell the 4 atelectatic areas. The opposite lungs, uninvolved with pneumonia, were normal. These findings suggest that a decrease in pulmonary surfactant may occur secondary to atelectasis due to noninfectious factors. A surfactant antagonist may be produced during pulmonary infection, and diffuse out into neighboring lung tissue, causing the atelectasis occasionally seen with pneumonia. This concept implies that other substances introduced into the alveoli may also result in atelectasis.