Feedback control of enzymes in the pathway for the biosynthesis of aspartate-family amino acids was examined usingGlycine maxcallus, suspension cultures, and seedlings to determine if the controls are different in the three systems. Aspartate kinase activity (EC 2.7.2.4) derived from soybean cell suspension cultures is 50% inhibited by 1.0 mML-threonine and 50% inhibited by 1.0 mML-lysine. In contrast, aspartate kinase activities from 3-day-old seedling cotyledons and from callus cultures grown from cell suspension cultures are inhibited approximately 70% by 1.0 mMthreonine and 30% by 1.0 mMlysine. These data and chromatographic data indicate that different ratios of a lysine-sensitive and a threonine-sensitive aspartate kinase enzyme can occur in soybean cotyledons and callus as compared with cell suspension cultures. Threonine (1 mM) inhibited homoserine dehydrogenase activities (EC 1.1.1.3) from cotyledon and cell suspension cultures approximately 67 ± 2% while the activity from callus was inhibited slightly more (77 ± 1%). Four forms of homoserine dehydrogenase activity were detected on electrophoretic gels containing cotyledon extracts, while only three of these forms of homoserine dehydrogenase activity were detected in extracts of callus and cell suspension cultures. The threonine-insensitive form disappeared, while the three threonine-sensitive forms were still present. Dihydrodipicolinate synthase activities (EC 4.2.1.52) from all three sources were at least 95% inhibited by 1.0 mMlysine. Only one form of this enzyme was detected on electrophoretic gels.