Lysine-sensitive aspartate kinase was purified over a 1000-fold from carrot cells grown in suspension culture. A novel staining method was developed to visualise aspartate kinase activity in gels after non-denaturing electrophoresis. Estimates of the M r of the enzyme by electrophoresis under non-denaturing conditions gave a value of 253 000. This was confirmed using gel filtration on Superose 6 and 12. Sucrose density gradient centrifugation gave an apparent M r of 100 000, a result attributed to dissociation of the higher molecular weight form. The isoelectric point of the enzyme was determined by chromatofocusing. In the presence of 0.1 mM lysine the isoelectric point was 4.43, but in the absence of lysine a value of 5.16 was obtained. The K m for aspartate was 2.35 mM and for ATP 0.60 mM. The value for ATP was obtained from preparation of the enzyme with virtually no contamination by ATPases. Inhibition of the enzyme by lysine was potentiated by S-adenosylmethionine in a synergistic manner. Of the range of other inhibitors tested, only Rose Bengal and p-chloromercuribenzoate gave significant inhibition of enzyme activity. Optimum conditions for storing the enzyme as a freeze-dried powder were also determined.