The histochemistry and ultrastructure of calcified cerebellar deposits described by Tonge et al. (1977) are reported. The deposits were located by electron microscopy in the walls of blood vessels outside the basement membrane and, in most lesions, consisted of short fibrillar material arranged in multiple lamellae. A number of nonlaminated small bodies were present also. The material coated the vessel walls discontinuously with major and minor protrusions into adjacent nervous tissue. Histochemical analysis detected the presence of sialopolysaccharides in the lesions in adults and in a case of plumbism in a child, with minor differences in the type of sialic acid. X-ray fluorescence analysis supported by histochemical data indicated that, initially, the calcium was bound to the sialic acid and that calcium phosphate appeared in the lesions at a later date. The authors conclude that the lesion is formed by elaboration of sialopolysaccharides at the site but the possibility was not excluded that the polysaccharide may have been derived from a transudate across the vessel wall.