The ultrastructure of pulmonary alveolar macrophages (PAM) was studied in smokers, non-smokers, and in patients with alveolar proteinosis, Boeck-sarcoidosis and heavy-metal coniosis. Cells were collected from therapeutic or diagnostic bronchial lavage-fluid. Typical alterations are reported for each disease studied. Diagnostic bronchial lavage has opened new perspectives in pulmonology. However, the study of the cellular constituents of the lavage requires in addition to morphology, biochemical and immunological techniques. The most frequently occurring cell type of the bronchial lavage is alveolar macrophage (AM, AMA, PAM). In the intact lung its number amounts to 15 X 10(9). These cells are 12-20 microns in diameter and when stained with Giemsa, dark blue cytoplasmic granules and cytoplasmic vacuoles are seen. They are PAS-positive and contain a negligible amount of lipids. Cytochemistry has shown several enzymes in PAM. With transmission electron microscopy a polymorphic, excentric nucleus and single membrane-bound cytoplasmic inclusions were demonstrated. The surface properties of PAM were studied by scanning electron microscopy. The aim of the present study was to give account of electron microscopic findings in therapeutic and diagnostic bronchial lavage.