Aspartate kinase was purified from maize cells to near homogeneity after five steps of purification including ammonium sulphate, anion exchange chromatography, gel filtration and polyacrylamide gel electrophoresis. Two peaks of aspartate kinase activity could be eluted from anion exchange chromatography columns (Fast Flow Q Sepharose and Mono Q HR 5/5) with 150 and 200 mM KCl. Both peaks showed sensitivity to -lysine and the second peak in elution order was also inhibited by S-adenosylmethionine alone. Gels stained for aspartate kinase activity showed one broad band sensitive to inhibition by lysine. This band was electroeluted from the gel and further analysis showed the presence of three protein bands. The least electronegative band on the gel was identified as aspartate kinase. A third form of aspartate kinase, inhibited by threonine, was identified by gel filtration. The native Mr of the two lysine-sensitive forms of the enzyme was ca 150 000 and the threonine-sensitive enzyme 180 000, values different from those previously reported. These results confirm the hypothesis that in higher plants, at least three isoenzymic forms of aspartate kinase are present, two sensitive to lysine, the other to threonine.