A method for electron microscopic cytochemical localization of aβ-thioglucosidase (myrosinase) has been developed. Since sulphate is one of the products of the hydrolysis of sinigrin by myrosinase, it was felt that if the incubation was carried out in the presence of Pb++-ions an insoluble precipitate of electron-dense PbSO4 would be formed at the reaction sites. Following formaldehyde fixation a few different cell organelles in the extreme root tip ofSinapis alba showed reaction specificity for myrosinase but following glutaraldehyde fixation the enzymatic activity was inhibited. Biochemical tests of the isolated enzyme showed complete inhibition of the myrosinase by glutaraldehyde. Variations in the substrate concentration and incubation time indicated that the enzyme was confined to the dilated cisternae of the endoplasmic reticulum and in a limited extent to the mitochondria.