A procedure for the preparative isolation of 1-S-[(1Z)-3-hydroxy-1-[(sulfooxy) imino]-4-pentenyl]-1-thio-beta-D-glucopyanose, potassium salt (progoitrin) from Brassica oleracea is reported. The major steps in this procedure were: (1) extraction of glucosinolates with methanol from Brassica oleracea;