Cytochrome c oxidase (CO) is the terminal electron carrier of the respiratory chain and is localized exclusively in the inner membrane of the mitochondria. Using a specific rabbit antiserum against bovine heart CO, mitochondria were specifically stained in sections of formalin-fixed paraffin-embedded human tissues by an immunoperoxidase method. The intensity of the immunostaining in different human tissues correlated mainly with the number of mitochondria and abundance of mitochondrial cristae, i.e., surface area of the inner mitochondrial membrane. Mitochondrion-rich cells, such as myocardial and gastric parietal cells, had very strong cytoplasmic staining. This technique was especially useful for the specific identification of oncocytes in normal tissues and in lesions composed of oncocytes. In addition, the immunoperoxidase method for CO makes possible retrospective investigations of lesions composed of oncocytes, since routine paraffin blocks of formalin-fixed tissue are quite suitable for such studies.