The lungs of patients with pulmonary alveolar proteinosis were lavaged with saline. The lavage effluents were examined for the presence of unusual components which might be considered characteristic of the disease. Soluble proteins were resolved by electrophoresis on polyacrylamide linear gradient (2.5 to 27 per cent acrylamide) gel slabs and compared with proteins found in the patients' sera and lavage effluents from nondiseased human lungs. Many proteins in lavage effluents from both diseased and nondiseased lungs appeared similar to those found in serum. Several proteins from diseased lungs were not present in serum nor lavage effluents from nondiseased lungs. A number of proteins present in lavage effluents from nondiseased lungs appeared to be absent from lungs with alveolar proteinosis. Particulate components of lavage effluents from diseased and normal lungs were examined and compared using the electron microscope. Many particulate components including degenerate and disintegrating cells, ultrastructurally abnormal alveolar macrophages, and myelin-like structures were present in lavage effluents from diseased lungs but absent from normal lungs. Much of the particulate material resembled cell debris. The insoluble accumulations present in the pulmonary alveoli and airways of patients with alveolar proteinosis do not come from serum leakage nor from hypersecretion of materials normally found in these regions of the lungs. The major insoluble components appear to arise from cellular disintegration and the extracellular formation of myelin-like structures.