Mouse alveolar macrophages (AM) cultured in the absence of serum secrete an elastolytic enzyme. The elastase from AM resembles the previously described elastase from peritoneal macrophages (PM) in pH optimum and inhibition profile. The macrophage enzymes do not appear to be stored, and with periodic changes in the culture medium, accumulate extracellularly for up to 10 days. Resident PM produce barely detectable levels of extracellular elastase unless given a phagocytic load. Thioglycollate-stimulated peritoneal exudative macrophages (PEM), however, secrete easily detectable levels of elastase, which can be further increased with a phagocytic load. Without any additional stimulation, AM secrete an elastolytic activity comparable to that of the PEM receiving a phagocytic load, but unlike PM they do not increase elastase secretion after phagocytosis.