The light microscopic morphology, chemical composition, ultrasonic fractionation, ultracentrifugal separation and light absorption characteristics of Auer bodies have all been described. However, since the initial observations by John Auer in 1906 and until the advent of electron microscopy, few data have been added to elucidate the structure of these bodies. With the aid of the electron microscope, Auer bodies have been found to be coacervates of laminated, homogeneous, crystalline plaques with the long axis of the plaques in the same plane as the long axis of the rod. The ultracytologic resemblance between Auer bodies and cytoplasmic granulation supports previous histochemical evidence suggesting the genesis of Auer bodies from cytoplasmic granules. Existing evidence indicates that the newly discovered fibrillar formation and the recently described granule-vacuole body are the same structure. Although histochemical studies show similarities between these structures and Auer bodies, they are architecturally unrelated and probably have distinctly different pathophysiologic significance.