The experimental administration of carbon particles into the lung of 2 amphibian species (Bufo fowleri, Kaloula pulchra) induces the appearance of macrophages in the alveoli, which evidently are derived from blood monocytes. These cells are characterized by high activities of acid phosphatase and alpha-naphthyl acetate esterase. They take up the carbon particles into membrane bound vacuoles of variable sizes. In addition carbon was found in lysosomal bodies and free in the cytoplasm. Large accumulations of intraalveolar carbon are surrounded by 2 to 3 layers of flattened macrophages. These cells can multiply by mitosis. Laden with carbon they can disintegrate or return into the connective tissue underneath the alveolar epithelium. This epithelium exhibits a marked activation in areas of intraalveolar carbon deposits: increase in volume, formation of a well developed brush border, generation of a large nucleolus and numerous cisterns of the rough ER. The epithelial cells ingest carbon particles which can be found within clear vacuoles or lamellated bodies. Activated epithelial cells can undergo mitosis.