Cytological, cytochemical and immunopathological investigations were carried out at lung washings in a case of pulmonary alveolar proteinosis. There was an increased activity of alveolar epithelial cells of type II (AEZ II) with increased production of surfactant lipids. Abundant lipids of the surfactant system are also visible in the extracellular material rich in protein and lipids. Alveolar macrophages showed increased enzymatic activity with high levels of acid phosphatase and nonspecific esterase, but no phagocytosis of surfactant lipids. Immunopathological investigations exhibit a high content of IgG, IgM and IgA in the exudate. These findings demonstrate, that exudation of blood serum in alveoli and overproduction of surfactant material are two important processes in the formal genesis of alveolar proteinosis, dependent on each other.