Alveolar macrophages were removed from male F-344 rats by bronchoalveolar lavage and maintained in vitro for 14 days. Over this period the macrophages remained viable as judged by propidium iodide exclusion, lysosomal uptake of acridine orange, and phagocytosis of 1.75-microns latex beads. After 7 and 14 days of culture the cells contained lipid droplets. The macrophages were shown to ingest and dissolve monodisperse 57Co3O4 particles, which were relatively insoluble in extracellular medium. The fraction of 57Co3O4 dissolved intracellularly was determined at intervals during the culture period. The mean dissolution rate (+/- SEM) was 0.36 +/- 0.02% per day for the F-344 rat. This was lower than 1.4 +/- 0.05% per day estimated by Kreyling in macrophages from the beagle dog. The significance of this difference is discussed.