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Quantitative birefringency, histochemical and enzymatic studies were performed on cerebellar myoclonus bodies in 2 male cases of myoclonus body disease (Unverricht-type). The undifferentiated myoclonus bodies (Type I) showed extremely weak Brewster crosses withpositive sign. The cores of stratified bodies (Type II) exhibitednegative polarization crosses, which is in contrast to corpora amylacea and starch granules. In the outer zone of type II bodies birefringency was present only in radial structures. The optical anisotropy of myoclonus bodies is related to a-1,4-polyglucosan. The protein component and probably also acid groups do not participate in the total birefringency. The birefringency is caused by polyglucosan chains positively birefringent along the longitudinal axis. These chains are orientated radially in bodies of type I as well as in the outer zone of type II bodies; they are tangentially orientated in the core of type II bodies. The fibrils demonstrable by electron microscopy are likely to represent bundles of polyglucosan molecules. The biocrystalline pattern especially in the core of type II bodies is considered as a genetical feature of myoclonus body disease.