Peripheral nerve abnormalities in metachromatic leucodystrophy have been noted many times since Jacobi (1947) first reported metachromatic material in the sciatic nerve of her patient. The finding of metachromatic granules in a peripheral nerve biopsy, usually the sural nerve, has been recommended as a method of diagnosing this disease (Thieffry and Lyon, 1959;Hagberg, Sourander, and Thoren, 1962). The occurence of granules, mainly in Schwann cells, and also in macrophages, has been thought to be due to the same metabolic anomaly affecting the cells responsible for myelin formation: oligodendroglia in the central nervous system, and Schwann cells in the peripheral nervous system. Phase-contrast and electron microscope studies (Webster, 1962) and electrophysiological observations (Fullerton, 1964) NERVE TEASING This was done by a method modified from Thomas's (1955) account. Formalin-fixed nerves were washed in water and placed in 1 % aqueous osmic acid for 24 hours. After further washing, they were placed in 60 % glycerol in water until required. Under a dissecting microscope, at a magnification of x 25 or x 50, single nerve fibres were teased out freehand from fascicles, using fine sewing needles for dissection. Individual fibres were placed on clean slides in batches of about 20, straightened, dried in air at 37'C., and mounted in glycerine jelly. Some specimens of nerves after fixation were stained in bulk by the Hirsch-Peiffer method, by immersion in acidified cresyl violet for 24 hours, and then macerated in 60 % glycerine before being teased.