Ultrastructural screening of human breast tumor biopsies has revealed, in 43% of the carcinomas, and in 16% of the benign lesions examined. the presence of rod-like inclusions in mitochondria of epithelial cells. The inclusions are cylindrical in cross section, with a diameter of about 50 nm, and show regular striations in longitudinal sections. They are probably derived from cristae. Mitochondria with inclusions are usually enlarged and are found most frequently in epithelial cells of invasive tumors and near zones of tumor necrosis. Similar although not identical inclusions have been observed by other investigators in the liver of women with hydatidiform mole or choriocarcinoma, of pregnant women, and of women taking oral contraceptives. Identical inclusions have been found in mitochondria of HeLa cells treated with anti-HeLa serum. Although no clear-cut correlations were found between the finding of inclusions and parameters such as age of patients, nuclear grade or lymphocytic infiltration of the breast tumors, it is conceivable that the presence of mitochondrial inclusions reflects the effect of hormonal or of immunological factors.