The multisubunit enzyme aspartate transcarbamylase consists of six copies of two types of polypeptide chains, catalytic (C) and regulatory (R). A complex formed by the partial dissociation of this enzyme has been isolated. This species, which has the structure C(6)R(4), is a likely intermediate in the stepwise dissociation of aspartate transcarbamylase induced by mercurials. The formation of the complex is the result of the release of a single regulatory dimer (R(2)) from the parent molecule.The specific activity of the intermediate is essentially the same as that of aspartate transcarbamylase. By contrast, both homotropic and heterotropic interactions are reduced, but not abolished. These observations suggest that the allosteric transitions involved in the control mechanisms do not require the intact structure C(6)R(6).