Peroxidase activity was studied in alveolar macrophages and compared to the peroxidase activity in polymorphonuclear leukocytes using cytochemical techniques. A dense reaction product for peroxidase was observed in the primary lysosomes of polymorphonuclear leukocytes, but no significant peroxidase or peroxidative enzymes could be detected in rabbit alveolar macrophages. Furthermore, following vigorous phagocytosis of zymosan particles by alveolar macrophages in vitro, no peroxidase could be detected in association with the phagocytic vacuole. Exogenous horseradish peroxidase was ingested readily by alveolar macrophages so that abundant reaction product was demonstrated in pinocytotic vesicles and phagocytic vacuoles. The uptake of exogenous peroxidase by pinocytosis appeared to be more vigorous in alveolar macrophages than in polymorphonuclear leukocytes. These studies demonstrate that alveolar macrophages do not contain significant quantities of peroxidase and suggest that, it contrast to polymorphonuclear leukocytes, peroxidative metabolism does not contribute in a major way to microbial killing by alveolar macrophages.