The sizes of the alveolar macrophage (AM) and interstitial macrophage (IM) populations in the lungs of adult Fischer-344 rats were determined during steady state. AM labeled with opsonized erythrocytes during an in situ phagocytic assay were lavaged from excised lungs. The lungs were then dispersed into single-cell suspensions with collagenase and mechanical agitation, and the remaining mononuclear phagocytes were identified following a second labeling step. The size of the AM population was 1.3 X 10(7) cells, or approximately equal to 3% of the total lung cell population. The AM were negative for cytoplasmic myeloperoxidase granules. The size of the IM population was 8 X 10(6) cells, or approximately equal to 2% of the total lung cell population. IM were also negative for myeloperoxidase and, like AM, demonstrated marked Fc gamma R-mediated phagocytic activity. The high cell yields (approximately equal to 4 X 10(8) cells/lung; viability, greater than 85%) and the percentages of type II cells (11%) and ciliated epithelial cells (less than 0.5%) indicated the enzymatic dispersion method resulted in a highly efficient and representative sampling of the lung parenchyma. The collagenase method used in this study to disperse the lung cells into single-cell suspensions, in conjunction with additional cell separation techniques, may be of potential use for isolating enriched populations of IM, as well as other lung cell types, for in vitro study.