Electron-dense particles 80–100 nm in diameter were observed in electron micrographs of mitochondria in transformed mouse fibroblasts and in a human amnion cell line infected with supernatants of a human mammary carcinoma cell culture. Intramitochondrial particles were more numerous in the human cell cultures early after subculture. These particles were morphologically similar to those reported in mitochondria of Rous sarcoma virusinfected cells.