Abstract Native Escherichia coli aspartate transcarbamylase activity is stimulated at high values of the adenylate energy charge. This response interacts with inhibition by the endproduct CTP in the way expected for a regulatory biosynthetic enzyme. Activity of the dissociated catalytic subunit, however, is strongly inhibited by ATP. Thus an appropriate response to adenylates appears to depend on subunit interactions in the native enzyme. Magnesium ion sharpens the energy charge response of the native enzyme, but decreases the effect of ATP on the catalytic subunit.