Brassica juncea Coss. and Czern. is an important cultivated mustard species. It is said to be a native of Asia and is grown particularly in China, Japan, India, Pakistan, Nepal and eastern Europe as a vegetable and an oil-seed crop, although it has also been introduced into North America and certain West European countries as an oil-seed crop and for use in the manufacture of condiments5. The species is of botanical interest in that it shows marked variation in external morphology, especially leaf form1. Variation, too, has been reported in the type of volatile oil produced on the hydrolysis of the seed glucoside; three conditions have been described: (a) the production of allyl isothiocyanate only2,4; (b) the production of 3-butenyl (= ‘crotonyl’) isothiocyanate only4; (c) the production of a mixture of allyl and 3-butenyl isothiocyanates2.