Lungs from rodents, lagomorphs and primates were briefly fixed in purified glutaraldehyde and incubated with diaminobenzidene and peroxide at pH 7.6 for the demonstration of peroxidase activity and at pH 9.0 for the demonstration of peroxidatic activity of catalase. Great alveolar cells of all animals except the rabbit contained round to elongated microbodies that stained at pH 9.0. In mice, rough and smooth endoplasmic reticulum and the perinuclear cisternae of these cells were also stained. At pH 7.6 there was no staining of great alveolar cells in any species, except in mice, where a light staining of the endoplasmic reticulum, perinuclear cisternae and microbodies persisted. In rodents, microbodies ranged in diameter from 0.13 µ in mice to 0.22 µ in guinea pigs. In monkeys they measured approximately 0.15 µ. Microbodies were not identified with certainty in rabbit great alveolar cells. In rodents the ratio of microbodies to mitochondria was roughly 1:1, whereas in primates it was roughly 1:2. Using appropriate inhibitors it was concluded that staining at pH 9.0 was due to peroxidatic activity of catalase within peroxisomes. Extraperoxisomal staining in mice was attributed to endogenous peroxidase.