Intra-alveolar material from three patients with pulmonary alveolar proteinosis (PAP) who underwent therapeutic pulmonary lavage was studied to determine its composition and surface properties. This material was compared with material lavaged from "normal" lungs obtained from patients dying of acute nonpulmonary causes. The intra-alveolar material from PAP patients contains lipid and protein. Classes of lipids, their percent distribution, and distribution of the individual phospholipids were the same in PAP-patient material and controls. Serum proteins were found in amounts characteristic of normal serum. The PAP material exhibited decreased surface activity; after exposure to ethyl alcohol, normal surface activity was recorded. The intra-alveolar material in pulmonary alveolar proteinosis represents an accumulation of modified normal surfactant and serum, rather than abnormal lipids or proteins, or disproportionate quantities of normal lipids.