The distribution of alveolar macrophages, Type II cells, and alveolar septal connective tissue discontinuities or gaps in two neighboring alveoli from a human lung obtained at surgery, and preserved by vascular perfusion-fixation, was studied by electron microscopy. Discontinuities or gaps are defined as complete interruptions of all the connective tissue elements of the alveolar septum, including the basement membranes. Serial-sectioning of the alveoli, and the creation of montages of the entire circumference of each alveolus at intervals of every twentieth section (approximately 1.6 micron) at a magnification of X 2,160 permitted precise identification of cells and connective tissue gaps and allowed the reconstruction, by computer techniques, of the alveolar walls in 3 dimensions. These studies showed that all of the 48 alveolar macrophages identified, and over two thirds of all Type II cells and alveolar septal gaps, were located or bordered on alveolar septal junctional zones (within 10 microns of septal junctions). The profiles of 739 alveoli examined by light microscopy from 6 lungs similarly preserved by vascular perfusion-fixation, in which the alveolar surface lining was well fixed, also showed alveolar macrophages preferentially distributed in alveolar junctional zones. These were compared with 242 alveolar profiles from 3 other vascularly-fixed lungs and 971 alveolar profiles from 9 lungs fixed by way of the airways, in which the alveolar surface lining was lost. In these lungs, most of the alveolar macrophages were in the alveolar air spaces.(ABSTRACT TRUNCATED AT 250 WORDS)