Morphometric procedures have been used to study the characteristics of cells in the alveolar region of the lungs of rats, dogs, baboons, and humans. Compared with the other species, human lungs were found to contain greater numbers of macrophages and to have larger alveolar type II, endothelial, and interstitial cells. The thickness of the interstitium and the pulmonary capillary endothelium were also significantly greater in the human lungs. These differences in human lung anatomy may be due to increased exposure to airborne pollutants and to tobacco smoke. Despite the above differences and the fact that there are larger variations in size and functional characteristics of the lungs of these mammals, an overall striking similarity in characteristics of individual lung cells was found. The distribution of cells in alveolar tissue was remarkably constant between species as was the average volume and surface area of most cell types.