The isoenzyme pattern of chorismate mutase (EC 5.4.99.5) was examined by diethylaminoethyl-cellulose chromatography in a wide variety of plants. All plants contained a regulated form of chorismate mutase (CM-1), and most contained an additional, unregulated form (CM-2). The regulatory properties of CM-1 differed significantly between plants. Antisera prepared against CM-1 and CM-2 from Sorghum bicolor were used to test immunological cross reaction of chorismate mutases from other plants. There was a high degree of similarity between chorismate mutase isoenzymes from Sorghum bicolor and Zea mays and some with Hordeum vulgare, but all other species studied were antigenically distinct from sorghum. No homology between the structure of CM-1 and CM-2 was detected within any species.