Three aspartate kinase isoenzymes (EC 2.7.2.4) sensitive to lysine, threonine and S-adenosylmethionine isolated from maize cultures, have been studied in the presence of calcium and spinach calmodulin. Neither was able to produce any change in aspartate kinase activity. Apart from the normal inhibition caused by the addition of lysine or threonine, none of the calcium antagonists tested showed a significant inhibition of activity. These results confirm those previously described for carrot in which no evidence was obtained for a regulatory role of calcium or calmodulin on aspartate kinase activity.