Ultrastructural identification of light microscopic giant mitochondria was performed on the same specimens for light and electron microscopic observations. The liver tissue specimens were fixed in OsO4, embedded in epoxy resin, cut 4 microns thick and stained with polychrome. At the beginning of the study a light microscopic observation was made, and a microphotograph was taken. The identification of light microscopic giant mitochondria by conventional microscopy was identified by the occupation rate in liver cells, the negative findings of stainability and the morphological consistency (round, cigar-shaped and granular). The specimens were subsequently embedded again in epoxy resin and cut into ultrathin sections of 400 A. A transillumination electron microscope was used for the observation, and ultrastructural images of light microscopic giant mitochondria revealed that they were crystalloid bodies with a crystalline latticelike structure. The occupation rates within liver cells and the morphological shapes of the crystalloid bodies corresponded with those of light microscopic giant mitochondria. The light microscopic giant mitochondria obviously had different features from those of electron microscopic giant mitochondria and Mallory bodies (Yokoo's type II), although Mallory bodies showed the same staining properties as light microscopic giant mitochondria.