"Intramitochondrial filamentous bodies" (IMFB) were occasionally found within the matrix of some mitochondria of the thick limb of Henle of the rat kidney, but not elsewhere in the tubular system. Three types were recognized: type I, an accumulation of filaments 55 A thick; type II, a bundle of parallel filaments having the same thickness as those of type I and regular spacing, 87 A apart, from center to center; and type III, consisting of type II with regular light bands of 280 A periodicity and a helical border of prismatic tubular cristae. In addition to these, electron-opaque masses showing variable and faint substructures were found in the matrix of mitochondria. It is suggested that all these IMFB may originate from mitochondrial cristae and that type II IMFB may be an intermediate developmental form between type I and type III. After uranyl acetate staining, IMFB and the membranes of prismatic tubular cristae showed highly increased electron opacity. The literature has been reviewed for reports of intramitochondrial filamentous inclusions in various types of cells. These inclusions have been classified according to their structural characteristics and the localization in the mitochondria and compared with IMFB reported herein.

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